I know that "Commemorate" is a verb, but what part of speech is "Commemorates"? (What part of speech is it when you add the 's' to the end?)
Many languages including English, conjugate their verbs. This means that the verbs change their form according to various rules. In some languages, e.g. Spanish, verbs change much more than they do in English.
Here is the conjugation of the present tense of the verb "to commemorate" in English.
You (singular) commemorate
You (plural) commemorate
The only one that looks any different is the third one (he/she/it commemorates). It is the one that causes learners of English so much trouble. They simply forget to add the "s".
"commemorates" is the third person singular form of the present tense of the verb.