Expressing Desire: The 〜たい (~tai) Form

When you want to express a desire to DO something (like eat, sleep, or go), you attach 〜たい (~tai) to the verb stem.

Importantly, once you attach ~tai, the verb behaves exactly like an i-adjective. This means you conjugate it just like words like 'samui' (cold) or 'takai' (expensive).

Note: ~tai is only used for your OWN desires, or asking a direct question to someone else ('Do you want to...?'). You cannot use it to say 'He wants to...' directly.

水が飲みたいです is grammatically correct, but stated flatly to a stranger or in a formal context, it can sound a little blunt, closer to announcing a want than making a polite request. In service situations (a restaurant, a shop), Japanese speakers usually reach for a request form instead: 水をください or 水をお願いします, rather than describing their own desire out loud.

〜たい is most natural when someone has actually asked what you want, when you're talking to friends or family, or when you're describing your own feelings rather than asking someone to act on them. The moment the sentence is functioning as a request to another person, a request form is usually more natural than 〜たい.

The guide above covers 〜たがっている for observing that someone currently shows signs of wanting something. There's a second form worth knowing: 〜たがる (without いる) describes a general tendency, a person who characteristically wants X, not just right now. 子供はお菓子を食べたがる (Children tend to want sweets) describes a pattern, while 子供はお菓子を食べたがっている (The child is showing signs of wanting sweets) describes what's happening in this moment.

For JLPT purposes, 〜たがっている (the in-progress, observed-right-now form) is by far the more commonly tested pattern, so if you only have time to master one, prioritize that one first.