There is a misconception that building decks and playing decks are completely independent processes. There are distinctions between the two and you can be a good player but not a good deckbuilder or vice versa, but there is a little more to it. The gap between builder and player is meaningful. In this gap, translation occurs and intent is conveyed.
To help illustrate this, let's pull away from Magic and look at music. As most of you know, many hit songs are not written by the artist that performs it, but someone else entirely. Often, by someone who writes songs for a living and sells them to record companies, who then distribute them to artists as they see fit.
If the singer gets a song that is sad but sings the song in an upbeat and excited way, it really doesn't matter how good of a vocalist or how good of a song it is, as there was an error in translation. The songwriter has intent for this song so even though they might not be singing it, the lack of execution on that intent would be obvious. Songs are not able to be conveyed in a vacuum because there is more to a song than just the words being sung. In Magic, there is more to a deck than just the cards within it.
Let's be really drastic here and tell a story of a player who receives a tier-one deck, but has no knowledge of anything beyond that. They don't know the metagame or the way their deck is intended to be played. Now imagine them piloting it in round one. You see the player casting Rite of Flame on turn 2 in order to cast a Simian Spirit Guide. The player bottoms every card they see with Preordain as they just want to find another creature to keep beating down.
This player has no clue that they are holding a combo deck with a hyper-specific game plan. They are playing they deck just like they would most Limited or Standard decks. Intent, or a formal game plan, was never given to the player, so despite having a tier-one deck, they crash and burn.
It should not take many games of staggering over yourself before realizing that the deck in your hands has a unique game plan. Even if you can't piece together what that game plan is, you can still understand that you have been doing something wrong. But what about when the feedback is not as clear? For example, what if you played any of the four-color decks in the format but did not understand the general plan for developing your mana early? You could play hundreds of games without noticing the small percentages you gave up by fetching incorrectly in the early game. Casting Siege Rhino before Thunderbreak Regent seemed harmless enough, but maybe it eliminated a line of play in the process. Maybe you can no longer top deck a Kolaghan, the Storm's Fury for lethal.
These mistakes are often so concealed that they can be difficult to spot. Maybe they never come back to punish you, making them even more difficult to recognize. With more practice and a better idea of the intentions of the deck though, we can minimize the number of these mistakes that we make.
Really, the only way to ensure there are no errors in translation is to "design" the deck you are playing. I put design in quotes because you don't need to create an entirely new list to design a deck in this case. What you need to do is become as intimate with the deck as though you had designed it. This can come from the fact that you literally designed it, or it can come from you putting forth a lot of time and practice with another person's list. Usually during this time, you will make some small tweaks to the list and actually take a part in its design, but even if you end up running the exact 75 you started with, you should have learned all sorts of things about the deck. You should be more familiar with niche scenarios and alternative lines of play that are not obvious.
This process allows you to make deeper or more involved plays. You understand the interaction of Wasteland Strangler and Warping Wail, for example, or you now know that Kalitas, Traitor of Ghet is so important to your strategy that you should only play him when you have a removal spell to follow up.
It is not impossible to see these things while playing the deck for the first time with no practice or familiarity, but the chances you do are so much lower. During that time you should be focusing on life totals and combat math, not learning the interactions of your deck for the first time.
Of course, it is not always possible to take the time to learn a deck as though you had designed it. For those tournaments, there are some steps and precautions you can take to give you the best chance possible at success.
If you are ever looking for a fun exercise to demonstrate this, turn to limited. Do a draft or a sealed as normal and play with the deck. Have a friend draft or build a sealed deck without you around. Now try playing with that deck without having looked at it. In most cases, you'll feel the tension that comes with playing a deck without realizing its underlying plan.
Many people do not have the time to invest into learning decks to this level of specificity. Even if that seems like the case, this is such an important step in the playing process that I would not advocate skipping it. Instead, your goal as a player who recognizes their weakness as time, is to choose decks with simple or familiar game plans.
If you find yourself without a ton of time to prepare for a tournament, keep these things in mind when choosing a deck:
Lack of deck knowledge or lack of knowledge of intent usually comes around because the player did not have the time or desire to learn more. Obviously, if you don't have the desire to, then this article is probably not for you, but if you lacked time, there are some ways to sidestep this problem in the future.
One of my favorite ways is to become an expert at a certain style of deck. Imagine that there is an upcoming Modern tournament this weekend and you don't know what to play. You have little time invested into anything and don't see that changing. What do you play? Well, as we just discussed, something like Mono Red Burn or Affinity might be your best bet as they are proactive and fairly well-known strategies. But what if I took this same scenario and then noted that you played Abzan Company in your last five Modern tournaments?
Now, when I ask you what you want to play, the spectrum of decks is expanded from proactive, relatively simple decks, to any deck you are comfortable/familiar with, which in this case is Abzan Company. Even if Abzan Company is not the best choice for the tournament in a vacuum, for you personally, you will probably have better results with it than anything else based off of experience alone. You know rough sideboard plans. You understand matchups. Any percentage that raw power level might have give you by playing Eldrazi or something along those lines would be gained here in different ways. You would be more comfortable, better able to make correct decisions, and able to focus on in-game issues rather than metagame strategy.
I'm not advocating that you become a one-trick pony who only plays Abzan Company, but having some specialty deck in your back pocket comes in handy for situations like these. This will most often apply to Modern and Legacy, formats where rotations do not happen and therefore you have the time to pick up experience with a single deck.
That said, this can also apply to archetypes, which helps you across all formats. For example, if I were to go build a deck from scratch right now and then play it immediately, I would best be served making a ramp/land death deck of some kind, or a graveyard-based strategy, or possibly Bant. Those three archetypes have been my bread and butter over the years. I simply understand the concepts and lines for those styles of decks best.
I can certainly play and build other decks just fine, but if I were building and playing a deck with the fate of the world at stake, I simply have more of an edge with those decks. This comes from actual years of experience building and playing them.
If you can familiarize yourself with some archetypes like this, you have a much wider range of options should you find yourself building a deck last-minute.
In a world where we have data on decks and archetypes flying around constantly, it can be very easy to just pick up a good deck and expect to do well with it. After all, other people did well with it, so why not you? But a decklist does not tell the whole story and it is your job to discover the things about that list that are beneath the surface. If you are not doing this, you should try it sometime to see the difference. I know that when I feel mastery over a deck, I have a better time playing and do better with the list as well.
I have found success with dozens of decks that I brewed up the day or night before a tournament because I was doing the brewing. My theorycrafting and thoughts behind my card choices all help me to have a better understanding of my list and to therefore execute on its intent better.
Whatever method you choose to get better in tune with the list you're playing is worth it, so take some time and do some translating. The song will sound a heck of a lot better that way. Thanks for reading!