Hi, Melanie
You posted:
"i want to know if i start a sentence with "THE girls mean(or what ever word)" should i use an s after that word(mean/verb) or not."
"The" is an article in English, and it's an "invariable" word: you will use "the" with singular and plural nouns, and with feminine and masculine nouns, and also with mass and count nouns. The article does not determine subject-verb agreement.
In the sentence "The girls mean", "the girls" is the subject, and the most important word in the subject is the noun "girls". So, "girls" is what has to agree with the verb.
In order to see if you have to add an "s" to a verb in the simple present, you only need to find out which personal pronoun can replace "girls" (or "the girls").
The right pronoun is "they", so the form of the verb corresponding to "they" is used. You said verbs take an "s" only for "he", "she" and "it"... so here you don't use the "s".
"THe girls mean..." is correct.
If you said "the girl" (only one girl), the pronoun for that girl would be "she", so you wou;d need to add an "s" to the verb:
"The girl means..."
I hope this helps
Miriam