Learn Finnish: Lähetin eilen kaksi postikorttia. - I sent two postcards yesterday.

Lähettää is a Type 1 verb. To form the past tense, we first find the stem (lähetä-), where the double tt weakens to a single t. Then we add the past tense marker -i-. Because the final ä drops out before i in Type 1 verbs, we get läheti-. Finally, we add the personal ending -n for I, making lähetin.

Why is the word for postcard in the partitive form (postikorttia)?

In Finnish, any number greater than one requires the noun following it to be in the partitive singular form. Since the sentence has kaksi (two), the basic word postikortti takes the partitive ending -a to become postikorttia.

Where is the word for I (minä) in this sentence?

You don't always need it! For the first and second person (I, you, we, you plural), the verb ending already tells you who is acting. The -n on lähetin clearly means I sent, so it is very common to drop minä in everyday language.

Does the whole compound word postikortti change its form?