Noun case and plurality

Question

Take a sentence such as Я имею основания ей не верить.

If основания is accusative, then it must be plural. And thus the translation will be: I have reasons not to trust her.

But if основания is genitive, then it must be singular. And thus the translation will be: I have a reason not to trust her.

I'm sure native Russian speakers can identify the correct form instantly from years of input, but what is going on behind the scenes here?

Maybe I must simply know that there are no rules that make основания genitive here, and therefore it must be accusative, and mean reasons, not reason?

Answer 1

First, a side note: unlike most other languages, you don't use имею in Russian unless there's a reason to. See this question:

• Иметь vs у меня for physical things

In your case, you say

У меня есть основания ей не верить

Now, имею (or быть in proximal possessive sense) does not govern genitive when positive, but does when negative. That's how the language works. It is:

У меня есть + nom. (or я имею + acc.)

but

У меня нет (or я не имею) + gen.

Hence, you can say

У меня нет основания

which is negative, hence the object is in genitive, hence it is singular, and the whole sentence means "I don't have a reason"; or you can say

У меня есть основания

which is positive, hence the object is in nominative, hence it is plural. This means "I have (my) reasons".

Answer 2

You're correct - there are rules that make it clear. The Russian speaker instictively knows that the verb иметь takes the accusative and not the genitive case. However in a negative context, it sometimes does take the genitive:

Это не имеет смысла. [< смысл m. sense, meaning]

This doesn't make [lit: have] sense.

So to answer your question, what's going on behind the scenes here is a simple matter of case government, which can vary from verb to verb. For instance, the verb help takes a direct object in English, yet an indirect [dative] object in Russian. With this knowledge, the Russian sees Президент помогает бедным The president helps the poor and instantly knows that here бедным is the dative plural and not the instrumental singular, for example, which have identical endings in adjectives.