"You ate oranges."
Translation:आपने संतरे खाये ।
For transitive verbs (verbs which can have direct objects) in some tenses (simple past tense, perfect tenses etc), the verb conjugates with the gender and the number of its objects and not the subject.
In this case, we add a ने to the subject to specify that the verb is not conjugating with it (this type of construction is known as 'ergative').
So, 'My mother ate a roti' would be 'मेरी माँ ने रोटी खाई' because रोटी is feminine singular, 'My mother ate apples' would be 'मेरी माँ ने सेब खाये' because सेब is masculine plural etc.
Roughly half of all nouns are feminine and the other half are masculine so it is impossible to get a chart of them.
You can sometimes guess the gender based on things like word ending (words ending with ा are more likely to be male, those ending in ी are more likely to be female etc) or the category of the word (country names are usually male, rivers are usually female etc) but there are a lot of exceptions to each of these rules. The best way is that whenever you encounter a new Hindi word, you learn it with its gender. That is, instead of learning कुत्ता='dog', बिल्ली='cat', you learn मेरा कुत्ता='my dog', मेरी बिल्ली='my cat'.