adverbs - Can we use "then" before the main verb? "... we'll then go."

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but I'm always use then between to phrases (when it is mean: next or after that).

Let me finish this job, then we'll go.

My question is: Can we use it before tha man verb?

Let me finish this job, we'll then go.

Answer

Adverbs of time can be placed at the start of a sentence, after an auxiliary verb/modal, before a main verb or at the end of a sentence:

Soon we must make decision
we must soon make a decision
we must make a decision soon.

With so many choices of location, even small things affect the placement. In the case of your sentence, one of the things that may affect it is the rhythm. For this meaning, the word then must be a strong syllable, and in the first sentence go is also a strong syllable, giving a nice rhythm.

Let me finish this job, then we'll go.

The second sentence also requires go to be a strong syllable, which puts two strong syllables next to each other: we generally avoid this rhythm if possible.

Let me finish this job: we'll then go.

If we add some more information, for example a where clause, go loses its stress, which moves to the where clause, and we find this rhythm much more comfortable.

Let me finish this job: we'll then go and get something to eat.