Is Eroded Past Tense?

past tense of erode is eroded.

Is Rotten past tense?

The past tense of go rotten is went rotten. The third-person singular simple present indicative form of go rotten is goes rotten. The present participle of go rotten is going rotten.

Does Ankommen take Sein?

So technically, ankommen always builds its spoken past with sein, not with haben. However, ankommen is actually one of the verbs where you need both, spoken past and written past (preterit) in spoken German.