It's a good dog. It barks as soon as it ( to see ) a stranger

Question

It's a good dog. It barks as soon as it ( to see ) a stranger

Answer 1

It's a good dog. It barks as soon as it sees a stranger.

Answer 2

...as it sees a stranger.

Present simple, в третьем лице добавляем к глаголу окончание s.