"Jenter" is the indefinite plural.
a girl = ei jente (f), en jente (m)
the girl = jenta (f), jenten (m)
girls = jenter
the girls = jentene
For feminine nouns, the -a ending is only used for the definite singular.
For neuter nouns, the -a ending can be used for the definite plural, but not for the singular.
Masculine nouns do not have -a endings.
i'm relatively new to the language but i think it varies as a response depending on the question. like if you say "what does the girl eat" the response would be different from "what is the girl eating". the verb tense is different because "is eating" implies now, whereas "eats" implies anytime, just something the girl does. i guess it varies on context!