Breakdown of Ему страшно, потому что собака большая.
собака
dog
большой
big
потому что
because
ему
to him, to it
Questions & Answers about Ему страшно, потому что собака большая.
Why do we use Ему instead of Он for "He"?
This is an impersonal expression of state. In Russian, feelings like being cold, bored, or scared are often expressed as "to him it is scary." So we use the Dative case pronoun Ему (to him) instead of the Nominative subject Он (he).
What kind of word is страшно? Is it an adjective?
It looks like a neuter short adjective, but here it acts as a predicative adverb (sometimes called a state word). It doesn't describe a noun; instead, combined with the Dative pronoun, it describes the condition or feeling someone is experiencing.
Why are there no verbs in either half of this sentence?
In Russian, the present tense of the verb "to be" (is/am/are) is almost always dropped. That is why "he is scared" is simply Ему страшно and "the dog is big" is just собака большая. If this were in the past tense, you would see a form of the verb "to be" added.
Why does the adjective большая end in -ая here?
Большая describes собака (dog). Because собака is a feminine noun acting as the subject of its clause (Nominative case), the adjective must match it with the feminine nominative ending -ая.