Is the following sentence grammatical?
Secondly, doing research and inventing new things are only possible if someone has understood the concepts in that particular topic or area.
I'm concerned about the use of the verb to be. Grammarly claims that I should use "is" instead of "are". I am under the impression that I am listing two activities – doing research and inventing – and hence, I should use "are". Is it not similar to saying "X and Y are not feasible" or "X and Y are bad boys"?