"I saw elephants."
Translation:मैंने हाथी देखे ।
The plural of हाथी is हाथी itself because it is a masculine noun. However, in the oblique case (when it is followed by a postposition), the plural हाथी becomes हाथियों while the singular हाथी remains the same.
You can write 'I saw elephants' in two ways:
मैंने हाथी देखे । - where 'हाथी' is in the direct case and the form of the verb देखे tells you it is plural
मैंने हाथियों को देखा- where हाथियों is in the oblique case because of the postposition को
You can think of the two forms as being like 'see' and 'look at' respectively. However, keep in mind that when talking about seeing humans or pets, you use the second form ( को देखा) exclusively.
In the past tense, transitive verbs (verbs which can have direct objects) conjugate with their object(s).
Therefore, the verb देखना in this sentence must agree in gender and number with its object हाथी. हाथी is a masculine noun and in this sentence, it is used in the plural form. So, we use देखे.
If हाथी is used in the singular form, we need to use देखा.
मैंने हाथी देखा = I saw an elephant