Learn Finnish: Ostin hölmön hatun, mutta onneksi tuli säilytettyä kuitti. - I bought a silly hat, but fortunately I happened to keep the receipt.

This is the "inadvertent action" construction (vahinkorakenne), which implies doing something by chance, accidentally, or without a strict plan. It uses the 3rd person singular past tense of tulla (tuli) plus the passive past participle of the main verb in the partitive case (säilytettyä). Here, it translates best to "happened to keep" or "ended up keeping".

Why is the object kuitti in the nominative form instead of the accusative kuitin?

Because the tuli tehtyä construction is impersonal. In Finnish grammar, impersonal and passive structures do not take a genitive-looking accusative object. Instead, a whole object appears in the basic nominative case. Therefore, "the receipt" is simply kuitti.

Why isn't there a word for "I" before tuli säilytettyä?

This structure doesn't take a standard nominative subject like minä. If you want to explicitly state who did the action, you put the person in the allative case, such as minulle tuli säilytettyä (literally "it came to be kept unto me"). In this sentence, the "I" is already obvious from the first clause's verb ostin (I bought), so it is naturally left out.

How do we form the word säilytettyä from the dictionary form?