Dear N:

You wrote:

Are all these sentences correct? In which cases, does “unfortunately” apply to both clauses and in which cases only to the first?

1-Unfortunately, two people were injured; also, a few windows were broken.

This is correct. Because of the parallel construction of the two clauses, we would understand that “unfortunately” only went with the first clause and “also” went with the second clause.

2-Unfortunately, two people were injured; and also, a few windows were broken.

This would probably not be punctuated with a semicolon because of the conjunction “and.”

3-Unfortunately, two people were injured, and also, a few
windows were broken.

This is grammatical but probably would not be used in formal English. The same parallel construction with the adverbs as #1 applies.

4-Unfortunately, two people were injured; and a few windows were broken.

Same as #2.

5-Unfortunately, two people were injured, and a few windows were broken.

This is punctuated OK, and the “unfortunately” would be understood by most to apply to both clauses.

If you wanted to insure that the “Unfortunately” applied to both clauses, use a correlative conjunction like “not only…but also” or, perhaps, “both…and.” (“Both” and “two” together would sound awkward, though.)

Example – Unfortunately, not only were two people injured, but a few windows were broken also.

Or, to put more emphasis on the personal injuries rather than the broken windows:

Unfortunately, not only were a few windows broken, but two people were injured also.