Also, in other comments they said if its a "negative" sentence 是 is not needed..
Could the people you speak of have been referring to 很? I know that's the case for that character.
What are the rules for he plural vs she plural for they?
们 indicates it's plural! This applies for both 他 and 她! If the gender is unknown or mixed, use always 他.
(Sorry if i don't spell the pronounciations right) Ta men is plural and ta is singular.
You can't tell if the English sentence is talking about men or women....
I've made plenty of 'mistakes' while translating to Chinese because of this
I'm confused, as there is no way to know if the sentence is for plural he or she...
们 (Men) is used to make plurals.
她们 (Tāmen) = They (She plural)
他们 (Tāmen) = They (He plural)
我们 (Wǒmen) = We
你们 (Nǐmen) = You (plural)
That is correct, and in the same way, when a chinese says "tamen" you have to know from what is spoken if they mean men or women.
I exactly typed 他们 不是 老师。 Why are they showing it wrong???
That is correct -- I see you put spaces between words though. Unlike English, Chinese is written without spaces between successive characters and words.