Conjugation table for Japanese verb kau - to buy 買う   

The indicative mood is for ordinary, objective statements of fact. The present tense is used for incomplete habitual actions as well as for future intentions

Generally used to express probability, belief or intention.

Can also be used to express intention as in "Lets..."

For giving commands. Generally too abrupt for most situations other than telling off children or husbands but can be softened by adding "please" (kudasai)

The past indicative mood is actions completed in the past (I ate, I worked etc) and also for the equivalent of the present perfect in English (I have eaten, I have worked etc)

Generally used to express probability or belief regarding the past.

Used for past continuous action or state of being.

Used for action in progress, continuous action, state of being. Also used to express the same meaning as the English present perfect.

Unlike English it cannot be used for future intention (tomorrow I'm eating out, I'm going out later etc)

Used for the situations where "If" might be used in English.

Used for wide range of conditional and if meanings, past occurence, hypothesis etc

Expresses the idea of ability or cabability

Expresses the idea of making or causing someone to do something. My mum made me clean my room etc

It can also mean "let" or "allow". Please let me play!

The passive - like in English - is used when something is done to someone/something by someone/something else. My bag was stolen, He hit me etc

The passive can also be used in a slightly different way in Japanese to express when something regrettable happens to someone. Eg. rather than saying Fred's mother died (Fred san no okasan shinimashita) the passive could be used to express regret over the situation: Fred san wa okasan ni shinarimashita