Gender. All Old English nouns possess what is called "gender". That is, a noun will be masculine, feminine, or neuter. Don't confuse this kind of grammatical gender with biological gender. There is nothing biologically feminine about nouns which are grammatically feminine, nothing biologically masculine about nouns which are grammatically masculine, and nothing biologically neuter about nouns which are grammatically neuter. It's just that nouns have a feature which we call gender by convention. And this is a feature which cannot change in a noun. A noun may change its case or number, but a noun will never change its gender. This is a fixed feature, and you must be told what gender a noun is when you look it up in the dictionary. This is important to remember, because although the vast majority of a- declension nouns are masculine, not all of them are. You must memorise the gender of each noun as you would learn its meaning.