Learn Finnish: Lapsi odottaa suklaata. - The child is waiting for chocolate.

In Finnish, certain verbs always require their object (the thing the verb acts on) to be in the partitive case. Odottaa (to wait for) is one of these verbs.

Why does the partitive ending have a 't' (-ta) instead of just -a?

The basic dictionary word for chocolate is suklaa. Because it ends in two vowels, the partitive ending is -ta. If a word ends in a single short vowel, it usually just takes -a or -ä.

Do I need a separate word for "for" when saying "waiting for"?

No, the verb odottaa means "to wait for" all on its own. You don't need a separate preposition; the meaning is covered by combining the verb with the partitive case on suklaata.

Why is the ending -ta and not -tä?

This is due to vowel harmony. The word suklaa contains the vowels u and a, which belong to the back vowel group. Therefore, any ending added to it must also use the back vowel a.