I've heard it said that there is no rule for gender of nouns/verbs, and they are neutral. But others say that it is possible, such as vixen for fox.
How do I know which word to use in such cases?
The oldest form of the English language had grammatical gender, but lost it by the 13th century, except in pronouns: the only grammatical role played by gender today is determining what pronoun should be used to refer to an entity, and that is (with a very few exceptions, such as ships and countries) determined by “natural gender”. And even that distinction has been fading since the 1960s, when the feminist movement called attention to the discriminatory effect of using gendered pronouns.
We still deploy pronouns to distinguish between human and non-human, but where humans are involved we try strenuously to avoid distinguishing male and female except where that distinction is topical. In the same way, nouns which distinguish female and male animals (cow/bull, mare/stallion) are still safe to use, as are nouns which distinguish female and male people (woman/man, girl/boy) when the distinction is immediately relevant. But use of male terms to embrace both women and men is deprecated now—for instance, we are called upon to speak of humankind rather than mankind, of a mail carrier rather than a mailman.
The use of suffixes to distinguish female and male animal and agent nouns has largely vanished, too. Vixen is a rare survival from Old English, representing fox with the feminizing suffix -in and associated alteration of the stem vowel. Spinster is another such; it employs the feminizing suffix -ster. Both of these suffixes were lost in ME. There has been some tendency to use the suffix -ess, of French origin, in their place. A few common words (lioness, tigress) came directly from French, and are common; but the suffix has never been entirely nativized. Most agent noun derivatives (authoress, doctoress) have always felt slangy or affected, and the few which have entered common use—actress, for instance—are now distasteful to most of us.
A vixen is a specifically female fox. It is not the female term for a fox. A fox is a fox is a fox. A vixen is a female fox. Just like a woman is a female human.
There are no genders for nouns and verbs in English like in other languages, unless you are specifically referring to a male or female species.
Nouns are only gendered if they refer to something gendered (and not necessarily then).
Some examples of when they are:
Nouns themselves changing
Gender of the pronoun you'd use to replace it
I can't think of any examples of verbs being gendered.