To be (past form) – I was in Berlin last week

All English sentences must have verbs and therefore to be is used to link the subject with its complement, e.g. I was in Berlin last week,
Alle englischen Sätze müssen wenigstens ein Verb aufweisen. Deswegen wird to be gebraucht, um das Subjekt mit dem prädikativen Ergänzung, z.B. I was a teacher.

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I was in Berlin last week.
He was in the office this morning.
We were in a meeting yesterday.

Negative sentences are formed by using not, e.g. I was not in the office yesterday.
Verneinungen werden mit not gebildet, z.B. I was not in the office yesterday. (“Ich war gestern nicht im Büro”).

Questions are formed by inverting the subject and verb, e.g. Were you in the office yesterday?
Fragen werden gebildet indem Subjekt und Verb vertauscht werden, z.B. Were you in the office yesterday? (“Warst du / waren Sie gestern im Büro?”)

Was not can be shortened to wasn’t and were not can be shortened to weren’t. We normally do this in speech and informal situations. The full form is used in formal writing.
Was not kann zu wasn’t und were not zu weren’t verkürzt werden. Normalerweise geschieht dies im gesprochenen und informellen Englisch. Die (ungekürzten) Langformen werden in förmlichem Schriftenglisch gebraucht.