Learn Finnish: Me olemme matkustaneet Aasiassa ja Afrikassa.

To build the perfect tense, you use the helper verb olla (to be) conjugated to match the subject, plus the past participle form of the main verb. Here, that is me olemme (we are) + matkustaneet (traveled).

Why does the main verb end in -neet (matkustaneet) instead of -nut?

Because the subject me (we) is plural. In the perfect tense, the main verb has to agree with the subject's number. If it were a single person, you would use the singular -nut ending (like hän on matkustanut), but for plurals, it changes to -neet.

Why does Afrikassa only have one 'k' when the continent's name is Afrikka?

This is a classic example of consonant gradation. Adding the -ssa ending closes the final syllable (A-fri-kas-sa), which weakens the strong double kk into a weak single k.

Why do Aasia and Afrikka take the -ssa ending?

The -ssa ending is the inessive case, which translates to 'in' or 'inside'. In Finnish, you generally use this inner location case when talking about being located in continents, countries, and most cities.