The two Estonian inifinitives

Estonian features a seconday infinitive, the 'da' infinitive.

While sööma is the regular (ma) infinitive for 'to eat'.

Süüa is the 'da' infinitive of the verb 'to eat'.

The 'ma' infinitive receives its name because words in the 'ma' infinitive form usually end in 'ma'

While words in the 'da' infinitive usually end in 'da'.

teadma 'to know', 'ma' infinitive.
teada 'to know' 'to find out', 'da' infinitive.

There are exceptions though.

olla is the 'da' infinitive form of the verb olema, and it doesn't end in 'da'.

näha is the 'da' infinitive form of the verb nägema, and this one too doesn't end in 'da' in its 'da' infinitive form.

Although pidama means 'to keep'.

In Estonian the verb pidama is also used to express 'to have to' and 'must'.

The 'da' infinitive is particularly used when the action has an hypothetical component, this means, it may or it may not occur.

ma pean sööma 'I must eat'

ma tahan süüa 'I want to eat'

The 'da' infinitive is used when expression wishes, intentions and possibilities, a clear example of a hypothetical component.

ma võin süüa kõike 'I can eat anything'

This expresses a possibility

The 'da' infitive is also used to express emotions, or feelings.

Ma armastan jalutada 'I love to walk' this expresses emotion.

The 'ma' infinitive in particular, is used after verbs of motion, like 'going', 'coming', 'driving'.

Ta läheb sööma 'he goes to eat' 'he is going to eat' here the 'da' infitive is used after a verb of motion.

Sitting does count as a verb of motion too, so in me istume laulma, the 'ma' infitive is used to express the action.