Learn Russian: Дом оказался слишком дорогим. - The house turned out to be too expensive.

The verb оказаться (to turn out to be) usually requires the descriptive adjective or noun that follows it to be in the Instrumental case. Since дом is masculine, the adjective takes the masculine Instrumental ending -им, making it дорогим.

What is the infinitive of the verb оказался?

The infinitive is оказаться, a perfective verb meaning "to turn out to be" or "to find oneself" somewhere. Because the subject (дом) is a masculine singular noun, the past tense form used is оказался.

Is it completely wrong to say Дом оказался слишком дорогой using the Nominative case?

You might actually hear native speakers use the Nominative case here in very casual, everyday speech. However, the Instrumental case (дорогим) is the standard, grammatically correct form, and it is what you should aim to use in your own speaking and writing.

Does the word слишком (too) need to change its ending to match the Instrumental adjective?

No, слишком is an adverb, meaning it is a fixed word that never changes form. It stays exactly the same regardless of the gender, number, or case of the words around it.