The -da infinitive

The second type of infinitive usually has the ending -da. In some cases, the suffix can be -ta or only -a (-d- has disappeared).

If the stem of the verb already ends in d or t, this consonant is normally not repeated and only -a is added to the end of the word.

When to use the -da infinitive?

1) after verbs expressing wish, intention, possibility:

• Ma tahan tantsida. --- I want to dance.
• Kas sa oskad tennist mängida? --- Do you know how to play tennis?
• Ma soovin puhata. --- I wish to rest

2) after verbs that express a feeling or emotion:

• ​Ma armastan meres ujuda. --- I love to swim in the sea.
• Mulle meeldib filme vaadata. --- I like to watch movies.
• Ma kardan üksi pimedas kõndida. --- I am afraid to walk alone in the dark.

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3) after impersonal expressions:

• On aeg koju minna. --- It is time to go home.
• Seda on valus vaadata. --- It is painful to watch this.
• Trenni on kasulik teha iga päev. --- It is beneficial to work out every day.

4) after the conjunction et, when it means 'in order to'

• Ma lähen poodi, et osta piima. --- I am going to the shop to buy milk.

5) when the infinitive is used as the subject in a sentence, or when it modifies a subject:

• Eksida on inimlik. --- To be mistaken is human. 

The gerund (-des form)

The -des form is derived from the -da infinitive and it is used to indicate an action which occurs at the same time as another.

It is constructed by substituting the endings -da, -ta or -a with -des, -tes or -es respectively.

• Käi poodi minnes pagariärist läbi. --- While going to the shop visit the bakery as well.
• Läksin koju joostes. --- I went home running.
• Nad istuvad vaikides. --- They sit (while) being quiet.