Learn Spanish (Latin America): La radiación habría alterado la muestra. - The radiation would have altered the sample.

It is formed by taking the conditional tense of the auxiliary verb haber (in this case, habría for the third-person singular subject la radiación) and adding the past participle of the main verb, alterado (altered). This gives us the literal meaning "would have altered".

Could habría alterado also be translated as "must have altered"?

Yes! A very common use of the conditional perfect in Spanish is to express probability, conjecture, or wondering about the past. Depending on the broader context of the conversation, this sentence could absolutely mean "The radiation must have altered the sample" or "The radiation probably altered the sample."

Why doesn't the ending of alterado change to feminine (alterada) to match la radiación or la muestra?

Because it is part of a compound verb tense using haber. When a past participle is used with any form of haber (like habría), it is locked in its default masculine singular form ending in -o. It would only change to alterada if it were functioning as an adjective (for example, la muestra alterada).

I've seen muestra used as a verb meaning "shows". How does it mean "sample" here?