Learn Croatian: Učitelj govori engleski. - The teacher speaks English.

The dictionary form (infinitive) is govoriti. Because it is a regular -iti verb, you make the "he/she/it" form simply by dropping the -ti from the end. This leaves you with govori (he speaks).

How would I change the verb govori to say "I speak English"?

For regular -iti verbs, the "I" form ends in -im. You take the infinitive govoriti, drop the -ti, and add -m, giving you govorim. You would say Govorim engleski.

Why is engleski written with a lowercase "e"?

Unlike in English, the names of languages are not capitalized in Croatian unless they are the very first word in a sentence.

Does učitelj refer to a male or female teacher?

Učitelj is the masculine word, so it refers to a male teacher. A female teacher would be učiteljica. The verb govori stays exactly the same for both.