The phrase “そんなものだと思っていたよ。” translates to “I thought it was like that.” or “I had thought so.” Let’s break it down:
“そんなものだ” (sonna mono da): “sonna” translates to “like that” or “so”. “Mono” is a generic term for “thing”, and “da” is the declarative form of the copula “desu”. So, “sonna mono da” means “It is like that”.
“と” (to): In this context, “to” is a quotation particle. It indicates that what comes before it is a quoted thought or spoken words.
“思っていた” (omotte ita): This is the past tense of “omotte iru”, which is the progressive form of the verb “omou”, meaning “to think”. So, “omotte ita” means “I was thinking” or “I had thought”.
“よ” (yo): This is a sentence-ending particle that adds emphasis or assertion.
So, putting it all together, “そんなものだと思っていたよ。” implies the speaker had previously thought or considered something to be a certain way. Depending on the context, this sentence could express surprise that their previous assumption was correct, or it could express resignation or acceptance that something is as they thought.