Questions & Answers about Я видел это собственными глазами.
The current lesson is about the pronoun сам. Why isn't a form of it used here for 'my own'?
Сам emphasizes who did the action, like saying 'I did it myself' (Я сам это сделал). Here, the word 'own' describes the eyes you used, not the fact that you did the seeing alone. When 'own' indicates possession or an attribute, you use свой or, for stronger emphasis like in this sentence, the adjective собственный.
Why is собственными глазами in the Instrumental case?
In Russian, when you use a tool, instrument, or body part to perform an action, you put it in the Instrumental case without a preposition. Just like you write with a pen (ручкой), you see with your eyes (глазами).
Could I just say своими глазами instead of собственными глазами?
Yes! Я видел это своими глазами is very common and perfectly natural. Using собственными (from the adjective собственный) just adds a bit more dramatic emphasis, similar to saying 'with my very own eyes' in English.
What is the base form (Nominative singular) of глазами?
The base form is глаз (eye). It has a slightly irregular plural in the Nominative case with a stressed ending: глаза (eyes). However, in the Instrumental plural used here, it takes the completely standard -ами ending to become глазами.