Learn Turkish: Sokakta aniden durduk ve çok şaşırdık. - We stopped suddenly on the street and were very surprised.

In the present tense, the we form is shown with suffixes like -uz or -ız. However, the past tense uses a completely different set of personal endings. For the we form in the past tense, you always just add -k right after the past tense suffix.

Why does the past tense suffix change from -du in durduk to -dı in şaşırdık?

This is 4-way vowel harmony at work. The verb root dur- has the vowel u, so the past tense suffix becomes -du. The root şaşır- has the vowel ı, so it takes -dı.

Why is the word for on the street sokakta and not sokakda?

The word sokak ends with a hard, voiceless consonant k. Because of consonant harmony, the d in the locative suffix -da hardens into a t, making it -ta.

Why don't we need the word biz at the beginning of the sentence?

In Turkish, the subject is already built into the verbs. The -k at the end of durduk and şaşırdık tells us exactly who did the action. You only need to add biz if you want to add special emphasis.