"Elephants eat big bananas."
Translation:हाथी बड़े केले खाते हैं।
hai/hain can be omitted if the sentence is negative, i.e. "Hathi bade kele nahi khate" is correct, but if the sentence is not negative then you have to use hai/hain after every verb, when it's conjugated in the present tense anyway, which this sentence is. The rules change a bit for past and future tense.