Here are the common mistakes of my Spanish-speaking correspondents:
1. adverbs in the wrong place. 'I have seen a lot this problem' or 'I want to speak fluently English' .
2. forgetting to put in the pronoun... is nice, yes? Is raining, today. Are good ideas!
3. 'The' in English is used a lot less frequently than in Spanish, so it's difficult for them to leave it out when talking about a general idea. 'As in: The life is hard and the death comes soon'.
4. If they don't watch it, adjectives get pluralized: many larges chairs, two cutes dogs, my basics errors.
Those would be the ones from off the top of my head. (For learning Spanish, it's the dreaded estar/ser and por/para monster, and all the multiple 'que'-s all over the place and the subjunctive)