is a phrase that behaves in almost exactly the same way as ～のが上手（じょうず）
, but rather than meaning ‘to be good at (A)’, it means ‘to be bad at (A)’, (which is a much more handy expression, if you are like me!). This particular expression is used with the plain (dictionary) form of verbs only.
I am bad at teaching kanji.
He is bad at driving.
In these examples, we can see that there is no difference between Ichidan verbs and Godan verbs. Both will appear in their plain form. In this expression, のが
is performing the same function as in the regular のは, and のが
constructions. This function is nominalization (turning a phrase into a noun). However, のは
cannot be used with this phrase, as が
is always required with adjectives.
If you want to express that you were bad at something (in the past), the verb will still remain in the plain form, but 下手（へた） will change to the past tense.