English is not my first language. I'm about to file Provisional application. But it might contain grammatical errors. I'm planning to hire a patent attorney for drafting the non provisional. Can I file the provisional now with grammatical errors and fix later in non provisional?
They are allowed and if they don't alter the content they are not a problem. Mistakes that make something unclear or change the meaning of the sentence should be avoided - they cannot be corrected when filing the non provisional.
If a change introduced with the non provisional introduces a substantial change, you lose the priority date for that changed content - and if the change is not substantial the mistake wasn't substantial. In that case it can be corrected but there is no need to - and introducing changes without need bears the danger of invalidation if a court later later finds the change to have been substantial.
See this question and its answer for more information. Maca explained in the answer why changes shouldn't be made, too. Spelling might be fine, grammar however is already dangerous.