Learn Croatian: Učim hrvatski dvije godine. - I have been learning Croatian for two years.

In Croatian, if an action started in the past and is still happening right now, you use the present tense. Since you are still currently learning Croatian, you simply use Učim.

Why is the number two written as dvije instead of dva?

The number two changes based on the gender of the noun it goes with. Since the dictionary form of the word for "year" (godina) is feminine, you must use the feminine form dvije. If you were counting a masculine or neuter noun, you would use dva.

Why is the word for year godine in this sentence?

After the numbers 2, 3, and 4, Croatian requires the noun to be in the genitive singular form. The genitive singular of the feminine noun godina is godine. Even though it happens to look exactly like the regular plural form, it is grammatically the genitive singular.

The English sentence says "for two years". Why isn't there a word for "for" in the Croatian sentence?

When expressing how long an ongoing action has been happening, Croatian doesn't use a preposition like "for". You just state the time phrase (dvije godine) directly in the sentence.