1
× She doesn’t works here.
√ She doesn’t work here.
× Does she works here?
√ Does she work here?
When we use does or doesn’t in present questions and negatives, the main verb of the sentence is used without the 3rd person -s, because the -s is already included in does.
2
× They didn’t helped me.
√ They didn’t help me.
× Did they helped you?
√ Did they help you?
When we use did or didn’t in past questions and negatives, the main verb of the sentence is used in the infinitive form (without -ed), because the past mark is already included in did.