Can "was" be followed by "how"?

Can "was" be followed with "how" in an English sentence. example "Thing I did not understand was how you did it".

Solution 1:

There is no problem with 'how' following 'was'. (You do have an error in that there should be either a definite or indefinite article in the first part of the sentence).

In the corrected version:

The thing I did not understand

is a phrase which is the subject of the sentence

was

is the verb, and

how you did it

is the object phrase. Your sentence is not structurally different from

The animal [subject phrase] was [verb] a lion [object phrase].