May I know which are grammatical in the following
It is often more important to know what you cannot do than knowing
what you can doIt is often more important to know what you cannot do than know what you can do
Above using know, my reasoning is previously I have used to know what…than know，but it seems not correct, but I am not sure why.
- It is often more important to know what you cannot do
than what you can do
Best Answer
There aren't special rules of grammar preventing you from writing clumsy sentences that arbitrarily switch between verb forms. It's purely a matter of style.
As implied there, it's bad style in example #1 to have a "contrast-based" sentence construction with an infinitive (to know) on one side, and a gerund (knowing) on the other. They don't "balance".
OP's example #2 is okay because it has infinitives on both sides (with the implicit to component elided the second time). Some people might be critical of that elision, precisely because it detracts from the "mirrored halves" construction. I'd take that line myself in the written format, but in speech I think it helps the prosody/flow - the speaker's in a hurry, because there's still a bunch more words before he gets to the all-important contrast of can with previously-specified cannot.
OP's example #3 just goes further with the elision (#4 could just delete the final do). Again, it's a stylistic choice of no real significance. Idiomatically I'm sure most people would use can't rather than cannot (it's not "informal" here), but context and personal preference come in many varieties.