"strengthen" is a weak verb.

Isn't it funny? "strengthen" is actually a 'weak' verb. Shouldn't that word be a 'strong' verb?

You have "strengthen", "strengthened", and "strengthened". Looks like a regular verb, or am I missing something?

"weak" verbs are 'weak' because they add a dental suffix to form the past tenses (preterite and perfect).

'strengthen' is thus "weak" as shown in the foregoing post.

yield/yold/yolden would be a byspel of a 'strong' workword, because it does not add a dental (either -d ot -t) to the stem.