Are these sentences OK?
He's been taken to hospital on suspicion of appendicitis.
This letter has a lot of spelling errors.
This letter containes a lot of spelling errors.

Question

Are these sentences OK?
He's been taken to hospital on suspicion of appendicitis.
This letter has a lot of spelling errors.  
This letter containes a lot of spelling errors.

Answer

'He has been taken to hospital with suspected appendicitis' is more common although your sentence is perfectly grammatical and understandable.
2nd sentence is fine
'contains', not 'containes'.  You can also say 'there are a lot of spelling errors in this letter'.