Learn Polish: Jego zachowanie było zupełnie poza normą. - His behavior was completely outside the norm.

The preposition poza means outside or beyond. When describing a static position or state (answering where?), it requires the instrumental case. Since his behavior was currently outside the norm rather than moving there, the feminine noun norma takes the instrumental ending to become normą.

Why is the verb było instead of był?

In the Polish past tense, the verb must match the gender of the subject. The subject here is zachowanie (behavior). Because it is a verbal noun ending in -nie, it is neuter, so the verb takes the neuter past tense form było.

Could I use całkowicie instead of zupełnie here?

Yes, both words mean completely or totally and are interchangeable in this context. Zupełnie is an excellent and very common choice here to emphasize that a situation or behavior is entirely out of the ordinary.

Is poza normą a common set phrase?

Yes, it translates directly to outside the norm or out of line. You will frequently hear poza normą used to describe inappropriate behavior, unusual medical test results, or statistics that break from the standard.