On this site http://german.about.com/library/anfang/blanfang15.htm nouns have different genders in plural forms, why?Also is there a site where I can check the gender of nouns?
Perhaps you should have a short look at the gender of nouns:
The gender (not to be confused with "sex") is a property of nouns.
The German language has three genders, male, female and neuter.
- Usually a noun has one specific gender,
- for some, two are permissible (e.g my pet peeve "das/die Schorle"),
- sometimes different genders denote different things, e.g. der Laster (Lorry) / das Laster (vice).
The gender can usually be determined by a quick look at the article (both definite or indefinite) or matching adjectives and their declension, but also every decent dictionary should always state the gender with the noun.
What might have led you to the assumption that nouns change their gender is the fact that the pattern for the use of articles article is not the same as in English (definite: "the", both singular and plural; indefinite: "a" for singular, none for plural).
Basically, the definite article changes from singular to plural, for plural the definite article is always "die", but should not be confused with feminine singular "die". Indefinite articles are dropped, similar to English.
male: der Tisch -> die Tische (but: ein Tisch -> Tische)
female: die Tasse -> die Tassen (but: eine Tasse -> Tassen)
neuter: das Ziel -> die Ziele (but: ein Ziel -> Ziele)
German nouns don't change gender in plural. The thing that confuses you is that the articles der/die/das have all three the same plural form die/die/die.
So it is der Mann, die Männer (masculine in singular and plural)
die Frau, die Frauen (femine in sg and pl)
das Kind, die Kinder (neuter in sg and pl).
The articles (der, die, das) go with singular objects. For plural, German uses the article "die". As a following the object seems to be of a different gender sometimes - but it isn't. It is just a double use of the same word I guess.
You can imagine it like the English "you" which can be both, plural or singular. The German "die" can be singular (only feminine) or plural, too :)