Breakdown of Он гордится обоими гибридами.
он
he / it (for a masculine noun) / it (for a neuter noun)
гордиться
to be proud
оба
both (m/n)
гибрид
the hybrid
Questions & Answers about Он гордится обоими гибридами.
Why do we use обоими instead of оба in this sentence?
The verb гордиться (to be proud of) always requires its object to be in the Instrumental case. Because гибрид (hybrid) is a masculine noun, we must use the masculine Instrumental form of 'both', which is обоими.
Why is there no word for 'of' translating 'proud of'?
Russian doesn't use a preposition here. The idea of 'of' is built directly into how the verb гордиться works: it connects to its object solely by putting that object into the Instrumental case (гибридами).
If he were proud of two feminine things, like two machines (машины), how would the word for 'both' change?
The feminine form of 'both' in the Instrumental case is обеими. So the sentence would become Он гордится обеими машинами. Notice the stem changes from обо- (masculine/neuter) to обе- (feminine).
Does обоими follow standard plural adjective endings?
Very closely! In all cases except the Nominative and Accusative, the forms of оба use standard plural adjective endings attached to the root обо-. Here, it takes the Instrumental plural ending -ими.