schwimmen

Schwimmen (to swim) is a strong verb, though its present tense gives no sign of it: ich schwimme, du schwimmst, er schwimmt, wir schwimmen, ihr schwimmt, sie schwimmen. The vowel changes only in the past. The simple past is schwamm and the past participle is geschwommen, so the stem vowel runs i, then a, then o across the three forms. That i-a-o pattern is shared with several other verbs, including beginnen (began, begun) and gewinnen (to win).

The auxiliary verb depends on what you mean. Use sein when the swimming covers a distance or has a destination: ich bin ans andere Ufer geschwommen (I swam to the other bank). Use haben when you are describing the activity itself with no direction involved: ich habe eine Stunde geschwommen (I swam for an hour). In practice sein is the more common choice, and it is the safer default if you are unsure.

To say you are going swimming, German pairs two verbs: schwimmen gehen, as in wir gehen morgen schwimmen (we are going swimming tomorrow).

Related words come up often. Das Schwimmbad is a swimming pool, der Schwimmer and die Schwimmerin are swimmers, and die Schwimmweste is a life jacket. The figurative use exists too: er schwimmt im Geld means he is rolling in money.