I read for a few screen agencies and never fail to be amazed by the dismal quality of spelling, grammar and punctuation in some scripts. If you can't spell, try using a spell-checker. Won't pick up instances where you've put in the wrong word spelled correctly, but it's a step in the right direction. Learning basic rules of punctuation and correct grammar wouldn't hurt either.
Screenplays are written in a very particular format. Commonly held wisdom is that one page of standard format screenplay equates to one minute of screen time, if written right. But the standardised format also serves another purpose - by making all screenplays look the same, it makes their formatting invisible. You see the story, not the layout on the page.
A script littered with spelling mistakes, wildly incorrect punctuation and grammar that would make a schoolmaster weep drives me crazy. Not because I'm a spelling Nazi, but because those basic mistakes keep reminding me I'm a reading a script. Any chance of being swept along by the story gets destroyed by stupid errors that could have been so easily avoided.
If you know you can't spell, are shaky on grammar and have problems distinguishing an apostrophe from a comma - ask a friend for help. If you're dyslexic, get someone to proof-read your screenplays before you submit them. There's no shame in admitting you need a little bit of assistance, no weakness in the fact you're not perfect at these common sticking points.
Trust me, I spent years mistyping the word problem [it was a real probelm for me] and my grasp of grammar is tentative at best. Before I send out an important document, I'll print it out and give the hard copy a proof-read. If it's really important, I'll ask a friend for a second read. Amazing how often your eyes miss an obvious error on screen.
Recently I had one character offering another a bride to look the other way. That would have sense in a story about arranged marriages, but not a narrative about ethics in academia. Another time I had somebody watering their lawn with a hope pipe. And let's not talk about the crowded prison cell that afforded no room for piracy [yo ho ho and a bottle of glum].
Do yourself a favour, scribes - proofread. Onwards!