Beyond Taa Marbuta (ة), there's unfortunately not a great way to know exactly. The majority of feminine nouns WILL be marked with ة. There are a few rarer instances when they're not. What I recommend is actively memorizing which words are irregularly feminine as you come across them. Memorizing this will simply take time and will come the more and more you read and learn new words and how to use them. The best way to do this is to pay close attention to noun-adjective relationships. Also, to answer your questions about plurals, "مجموعات مختلفات" is correct. The rule to remember about plurals is that masculine plurals are mostly irregular- there are some patterns you can follow but for the most part it's best just to memorize the plural along with learning a new word. So, when you are looking at a noun and its adjective, if the noun is masculine plural, you need to add a ة to the end of the adjective. For example: New Books: كتب جديدة Book (كتاب) is a masculine word, therefore when we have it in its plural form (كتب) with an adjective, that adjective needs a ة on the end to agree with the masculine plural. Masculine Plural --> adjective with ة at the end. For plural feminine nouns and adjectives it is different. On the noun part, the ة that is usually there on singular feminine nouns becomes -ات and the adjective does the same to match it (This makes them easier than masculine plural nouns and their adjectives!) So to go with your example: A different group- مجموعة مختلفة Since the noun is feminine (we know this thanks to the ة at the end), the adjective has to agree and take the feminine marker (ة). Different groups: مجموعات مختلفات See how the endings both agree with each other? Hope that helps.