What are common grammatical mistakes native Spanish speakers make when writing in English for the first time?

Using THE when is not necessary.

Using the -ing ending.

mistakes when conjugating verbs in the third person (forgetting the ending S: He/ she go, he/she work

Using the regular ending ED for the past tense, with verbs that are irregular.

Using OF instead of FROM (both are Spanish DE). Using IN instead of AT or ON. In general, the correct use of English prepositions is very difficult to any Spanish speaker (check all what I wrote above, and surely you'll find some examples of what I'm saying).

I'm not answering the question, but that's a good point. I always wondered if or how often the Spanish spoken was incorrectly. If that same guy that answered you, I'd like to ask him...do Spanish speakers have an "ebonics" or alternative style of speaking. Now, I'm not trying to be funny...I'm quite serious. It's just that I don't remember the teachers telling me about that in Spanish class....

Yes, double negative is one of them. I've been there.

Another one is "dream with something" instead of "dream of something."

Another one is to say "people is doing something" instead of "people are doing something." In this case the ones that are wrong are the english speakers, because people is singular, even if it is a group of persons.

I mean, in the case of the double negative I can accept that spanish has a logical mistake, but in the case of "people are" it's the english language the one that has a logical mistake.

edit: another one: saying 'is' instead of 'it's'

Lots of mistakes.