보통 has multiple meanings. It can be a noun and the meaning as a noun is 'Being very common or average, usual'.
보통 is also an adverb and the meaning as an adverb is 'usually'.
'보통 때' is a phrase of two different nouns ( 보통 + 때 ) and it always needs markers such as 는, 에는, 와 in a sentence.
e.g. ) 보통 때와 다른 반찬
cf) 저는 성적이 보통이에요. (noun) : meaning = average
저는 보통 7시에 일어나요. (adverb): meaning = usually
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