Thomas:
Please memorize this easy rule, and you will NEVER be confused again:
ALWAYS use the base form (the infinitive with no "to") after so-called "modal verbs":
I CAN do it. (cannot do)
I COULD do it. (couldn't do)
I MAY do it. (may not do)
I MIGHT do it.(might not do)
I MUST do it. (mustn't do)
I SHALL do it. (shall not do)
I SHOULD do it. (shouldn't do)
I WILL do it. (won't do)
I WOULD do it. (wouldn't do)