"Cows give milk."
Translation:गायें दूध देती हैं।
हेता is used in generalisations when there is no other verb except for है in the sentence. In other sentences, it is mostly a matter of context.
As far as the habitual aspect goes, that is the main function of the simple present tense so this sentence would be understood as a habitual action as it stands.
Yes. The rules to make plural words depend both on the word ending and the gender of the noun. For masculine nouns, we leave the noun unchanged unless it ends with an ा in which case we change the ा to an े. For feminine nouns, we add an ें / एँ to the singular form unless the noun ends with an ी in which case we change the ी to ियाँ