Learn Spanish (Latin America): El vecino habría oído el disparo. - The neighbor would have heard the gunshot.

Yes, exactly. While habría oído literally translates to "would have heard" (like in a conditional statement: "he would have heard it if he had been home"), the conditional perfect is frequently used in Spanish to express a guess, assumption, or probability about a past event. In this context, it often translates naturally to "The neighbor must have heard the gunshot" or "The neighbor probably heard the gunshot."

If I want to say "The neighbor must have heard the shot", why use the conditional perfect (habría oído) instead of the future perfect (habrá oído)?

Both are used for guessing about the past, but they connect to different timeframes. You use the future perfect (habrá oído) to guess about a recent past event that relates to the present ("He must have heard it just now"). You use the conditional perfect (habría oído) to guess about an event further back in the past, usually when the surrounding story is already told in a past tense ("He must have heard it back then").

Why does the past participle oído need an accent mark?

The accent mark breaks a diphthong (a combination of two vowels in one syllable). The participle oído has a strong vowel (o) followed by a weak vowel (i). Without the accent, those vowels would blend into a single syllable. The accent over the i forces it to be pronounced as a distinct, separate syllable: o-í-do.

Could I use el tiro instead of el disparo for "the gunshot"?