Part 0 was rather long. It had to be, because it was necessary to set the stage for talking about card advantage. You remember when I said that more options leads to victory? This is the concept that is at the core of card advantage. If this doesn't normally apply to you, then you're not building your decks right (and we'll go over that in the future). In this section, we will be discussing card advantage, hand advantage, field advantage (also called presence), and breaking everything down into smaller units.
Card Advantage is the total amount of cards you control different from the total amount of cards your opponent controls. If you have 2 cards in your hand, and 3 on the field, and your opponent has 1 in hand and 5 on the field, then your advantage is -1 ([2+3]-[1+5]). It's a very simple concept. In that situation, your opponent has more options available in total because their hand and field has one more card than your hand and field. When most players talk about card advantage, they might be referring to the simple plus or minus that an individual card gives, the hand alone, or the field alone. These are simply mistakes in nomenclature. By itself, the term refers to all of your cards. And the advantage part denotes a comparison with your opponents' cards. Card presence is the number you have not compared to the opponent.
Hand Advantage as you probably guessed, is the total number of cards in your hand different than your opponent's. Generally, it is a good idea to maintain as many cards in your hand as possible and cause your opponent to waste cards from their hand to make the difference even larger. By the laws of probability, you are more likely to have options to the given situations than they are if you have more cards in the hand. Using drawing cards that give you +1 or more is very nice for adding options. Using drawing cards that give +0 but change things around is also useful as it swaps non-options for usable options (like Allure, Moray of Greed, and Solar Recharge). Use of cards like Magical Mallet, Reload, Card Destruction, or other -1s, is not recommended unless you play infernity where hand size should always be 0 or you are accepting the loss because none of your options work. In that case, you may want to rethink your deck build too. Hand presence is the number of cards in your hand without comparing it to the opponent.
Field Advantage is what happens when you commit to the field. This is where the mind games get played and where the ruse is started. Advanced duelists know to hold off on committing to the field until they can drain the opponent's options. They also know that if an opponent is stupid enough to commit large numbers of cards, the tables can be turned quickly in their favor. If both players are smart, this becomes a Prisoner's Dilemma where neither wants to commit much to the field, but if neither do, then one will take advantage of the empty field. If you commit too much, then one will take advantage with large destruction. Be wary of this balance. Try to judge the current game and use your utility cards wisely. If you can save them for mid-game, once you exhaust your opponent's options and simplify the field, you can quickly gain field presence and therefore advantage. This is the concept which wins duels.
And now that I've talked about utility, aggro, control, card advantage, hand advantage, and field advantage, you should know about two other very important concepts. Tempo and Simplification.
Simplification is any time the total number of cards from both players decreases equally. An example of that is when dark hole is played and you have 1 monster and the opponent has 2. Both of you lose 2 cards and the game is simplified by 4. 4 total cards are now gone from the immediate use of the game and cause the game to become more simple and easier to predict. As the simplification continues (usually into mid-game), it's entirely possible to know everything that can and will happen if you are sufficiently knowledgeable or prepared. This can be a terrible thing if it results in topdecking while you have little field presence. It is the reverse if there is topdecking while you have high field presence. Nuking the field while there are few cards in the hand can be a saving move for you and a way to quickly simplify the game. Cards which almost always subtract as much from you as from your opponent are called "simplifiers". Like Smashing Ground. Some Simplifiers are also called Floaters, which I covered in the last part (cards that replace themselves a la Card Trooper). If your opponent tries to simplify the game, but you can still gain advantage, you pretty much have won the duel.
Tempo is the speed at which both players lose cards. If you are gaining cards, you obviously have a negative tempo. If your opponent is losing cards faster than you, they are said to be "playing to your tempo". This is a very important concept and many cards can change the tempo of the duel like Deck Devastation Virus. If you play this and your opponent continues to draw weak monsters, then you can gain on him/her very quickly. Setting you up for card advantage or early field simplification. If you can control the tempo of the duel, then you can generally win it. Play a few games and feel out how quickly you make yourself lose cards (and therefore options) versus how quickly your opponent is making you lose them. If you are causing yourself the most trouble, then adjust your deck and play style. If it's your opponent, then simply adjust your deck. If you play by the opponent's tempo, then you are almost sure to lose.