A few Latin nouns are weird. They don't follow the normal rules for cases and don't belong to any particular declension. They're loners. Rebels. They always look the same, no matter what case they're in. There are very, very few of these words, but they do exist, and some of them are used relatively often. Here are the ones you might come across in your reading:

fas - fate, divine law        nefas - sin, abomination        instar - likeness                mane - morning               nihil - nothing, none