Question

When we were in Europe on vacation, we usually flew on airplanes. However, sometimes we took the train.

Choose the correct version of the combined sentence. When we were in Europe on vacation, we usually flew on airplanes however, sometimes we took the train.
When we were in Europe on vacation, we usually flew on airplanes, however sometimes, we took the train.
When we were in Europe on vacation, we usually flew on airplanes; however, sometimes we took the train.
When we were in Europe on vacation; we usually flew on airplanes, however sometimes we took the train.

Answer 1

The correct version of these two sentences when combined is:

When we were in Europe on vacation, we usually flew on airplanes: however, sometimes we took the train. 

We apply the semi-colon to indicate that they have different ideas, and that they are clauses being joined into one sentence.