Learn Finnish: He seurustelivat, koska rakkaus oli suuri. - They dated, because the love was great.

The verb seurustella specifically means to be in a committed relationship or to be dating someone steadily. If you just want to say 'to go on a date,' you would use a phrase like käydä treffeillä.

I know iso and suuri both mean 'big', so why use suuri here?

While both mean 'big,' suuri is generally preferred for abstract concepts, intensity, or magnitude, such as love (rakkaus), emotion, or importance. Iso is mostly reserved for physical, concrete size, like a big building or a large dog.

How is the past tense built in the word seurustelivat?

The base verb is seurustella. To form the past tense for 'they' (he), you take the present stem seurustele-, change the final e to the past tense marker i, and add the third-person plural ending -vat. This gives you seurustelivat.

Does the word koska change the word order in the second half of the sentence?

No, it does not. Unlike some other European languages, Finnish subordinate clauses (like those starting with koska or että) keep the standard subject-verb word order. You just write rakkaus oli suuri exactly as if it were a standalone sentence.