Learn Finnish: Asuntolaina on suuri laina.

Both mean 'big,' but suuri often feels a bit more abstract, formal, or emphasizes magnitude, while iso is usually more physical and colloquial. For a financial concept like a loan, suuri is standard.

How is the word asuntolaina formed?

It is a compound word made of asunto (apartment or housing) and laina (loan). Finnish uses these closed compound words heavily for specific concepts. Similar useful words are opintolaina (student loan) and autolaina (car loan).

Why is it suuri laina in the nominative case instead of the partitive suurta lainaa?

In 'A is B' sentences using the verb olla (to be), the complement (the 'B' part) stays in the nominative case if it is a countable, individual thing like a loan. You would only use the partitive here if the word was an uncountable mass noun, like water or coffee.

If I want to apply this lesson's grammar and say 'I am looking for a mortgage from the bank', how does this vocabulary behave?

The verb etsiä (to search) requires the partitive case for the object being sought, so asuntolaina becomes asuntolainaa. Furthermore, as you are learning in this lesson, verbs of searching require 'from' cases (like the elative) for the location. So you would say: Etsin asuntolainaa pankista.