Minä kuulin kuiskauksen ulkona.

Minä means “I” and marks the subject. Finnish is a pro-drop language, so you can omit it when context is clear.
Example without pronoun: Kuulin kuiskauksen ulkona.
Including Minä adds clarity or emphasis (“I, personally, heard…”).

kuulin is the first-person singular imperfect (simple past) of kuulla (“to hear”).
Formation: take the stem kuul-, insert the past marker -i-, and add the personal ending -n → kuul-i-n.

kuiskauksen is the accusative (often called genitive) singular of kuiskaus (“whisper”).
With telic verbs like kuulla, a complete, specific object takes the accusative/genitive -n ending. It signals you heard that entire whisper.

Yes. kuiskausta is the partitive singular. It implies hearing “some whisper” in an indefinite or incomplete sense.
Using kuiskauksen (accusative) treats it as a definite, fully heard whisper.

ulkona is the essive/adverbial form (suffix -na/-nä) of ulko-. It functions as an adverb meaning “outside.”
Finnish uses cases instead of prepositions, so “in/at the outside” becomes simply ulkona.

Yes. Finnish word order is flexible. Examples:
Ulkona kuulin kuiskauksen. (Emphasizes that it was outside)
Kuiskauksen kuulin ulkona. (Emphasizes the whisper)

Imperfect (kuulin) describes a simple past event.
Perfect (olen kuullut) connects the past event to the present (“I have heard”), often implying relevance now. If you’re just stating “I heard…” in the past, use the imperfect.

kuulin: [ˈkuu.lin] – “kuu” like English “coo,” “lin” as in “lean” without the ‘ea.’
kuiskauksen: [ˈkui̯s.ka.­ku.en] – “kui” like in “quick” without the k, “skau” with “au” as in “loud,” ending “en.”