Estonian features two forms of Infinitives for verbs: the -ma Infinitive and the -da Infinitive. The difference is mostly based on distinction, and the language could have evolved with only one form, but ended up with two forms which have different uses and are necessary for grammatically correct and familiar-sounding Estonian.
The -da Infinitive is one aspect of a verb that cannot otherwise be derived from the -ma Infinitve or even the plain Negative form. It can only be predicted from prior knowledge, such as memory or intuition. The -da Infinitive obeys the EKI morphological type list. We can now list this form alongside the -ma Infinitive and Negative in vocabulary:
Many verbs tend to follow -ma -> -da transformation. The verbs listed here now have the . emphasis dot added to them. If you pay attention to the Overlong emphasis in Estonian, you can avoid the Estonian foreign accent to some degree.
While the semantic difference between the -ma and -da infinitive forms seems obvious to the native speaker, they find it difficult to articulate when pressed.  There is, however, a fairly straightforward explanation:  The -da form is used to express an action that has a hypothetical component, i.e. it may or may not actually occur.
-ma actually has 5 forms, of which the primary is the Nominative, which we've used so far to list verbs themselves, and as arguments to the verb "pidama" (to need to). The other 4 also correspond to 4 of the 14 cases. The -ma form(s) are used to describe the verb or action itself.
-da has two forms: the -da and the -des. The -des is used as "while", which acts as a kind of present participle, but doesn't act as an adjective like participles do, nt: ''Seda teades, ta pidi seda tegema'' - Knowing that, he/she had to do it.