Questions & Answers about Við göngum yfir þessa brú saman.
Because ganga is the infinitive: að ganga = to walk.
In the sentence, the subject is við = we, so the verb has to be in the 1st person plural present form:
- ég geng = I walk
- þú gengur = you walk
- hann/hún/það gengur = he/she/it walks
- við göngum = we walk
So Við göngum... means We walk... / We are walking...
That vowel change is just part of how this verb is conjugated. Icelandic has many common verbs whose stem changes in some forms.
So with ganga, you should learn the forms as they are, rather than expecting the vowel to stay the same all the time:
- að ganga
- ég geng
- við göngum
This is normal Icelandic verb behavior, not something special about this sentence.