Base 2 + tai

A very useful Base 2 ending is tai, which is used to show that you want to do something:

• Watashi wa kasa o kaitai. (I want to buy an umbrella.)
• Kodomotachi wa asobitai. (The children want to play.)
• Bob wa tempura o tabetai. (Bob wants to eat tempura.)
• Miki wa sono eiga o mitai. (Miki wants to see that movie.)

The above examples are plain forms. To make them polite, add desu: Watashi wa kasa o kaitai desu, etc.
Again, tai shows that you want to do something, and is not used when you want something. Accordingly, tai is only used with verbs, and is never used alone with an object. For example, you wouldn't say watashi wa inu o tai for "I want a dog." You would use the adjective hoshii and say, "Watashi wa inu ga hoshii."¹
Now, what if you don't want to do something? In that case, we use takunai. Again, add desu to make it polite. Let's make the examples above negative. We'll make the first two plain:

• Watashi wa kasa o kaitakunai. (I don't want to buy an umbrella.)
• Kodomotachi wa asobitakunai. (The children don't want to play.)

And the next two polite:

• Bob wa tempura o tabetakunai desu. (Bob doesn't want to eat tempura.)
• Miki wa sono eiga o mitakunai desu. (Miki doesn't want to see that movie.)

Another handy derivative is takereba, which is the conditional form of tai. Use it for "if (you) want to":

• Terebi o mitakereba, yuushoku o hayaku tabenasai. (If you want to watch TV, hurry and eat your dinner.)
• Shichiji no densha ni noritakereba, ashita hayaku okimashou. (If you want to make the 7:00 train, let's get up early tomorrow.)

Word Check

kasa: umbrella
kodomotachi: children²
eiga: movie
yuushoku: dinner
hayaku: early; fast, quickly
shichiji: 7:00 (shichi [seven] + ji ["o'clock"])
densha: train
noru: to ride
ashita: tomorrow
okiru: to get up
(Verbs are shown in their plain form.)

Notes

1. Although hoshii is used in place of a verb, it is actually an adjective and is used when objects are wanted. It is never used with the particle o, but with ga instead. For more exceptions like this, please see my section on the direct object indicator o on my page.
2. Kodomo means "child." Although tachi can be added to make the plural "children," please remember that tachi works with only a few select nouns, mainly those describing people or animals. There are no plural forms for other objects, which makes Japanese uncomplicated in that respect.