grammar - Be mindful of using vs be mindful of

I am really confused with these sentences. I don't know the technical explanation for these. I am studying English and I do not understand what's wrong with the sentences.

1. You just have to be mindful of using verbs in your sentences.


2. However, please be mindful of articles and word choices.

I was told that when should be used for the first sentences. Both sentences are grammatically correct, however, the second sentence is okay and parallel. Also, both are non-standard. Is this correct? Are there any other explanations for this? Also, what are the alternatives for mindful? Thank you for your help!