In an effort to improve your English quickly, you’ll want to try and focus on your weakest areas. There are certain grammatical points in English that Spanish people find easy to comprehend and remember. There are others, however, that are a continual issue.
One such grammatical issue is the use of modal verbs. Modal verbs have a category of their own because they behave differently than other verbs in English. Although all the characteristics of modal verbs are important to remember, Spanish people have the most trouble with one in particular: modal verbs are never followed by the word ‘to’.
In my experience, ‘must’ in particular is often mistakenly used with ‘to’. So, remember that you need to say “must keep” rather than “must to keep”.