If you're writing a sentence in French with two verbs, the first being l'imparfait, would the second one be in the infinitive? For example "Le peuple prehistoriques était intelligente parce que ils pouvaient ériger les monuments sans des machines" ? Is this correct?

Generally when there are two consecutive verbs, the second one is infinitive.

But not auxiliary + past participle.
Ex : Je dois partir (I must go)
Je suis venu (I came)
"Les peuples préhistoriques était intelligents ( parce que) car ils pouvaient ériger des monuments sans machines"

Les peuples préhistoriques étaient intelligents....

Sorry I forgot the plural of "étaient"

and also "parce qu'ils ouvaient eriger...

OR car ils pouvaient ériger...

Sra (aka Mme)

Yes, your sentence is correct in terms of using the imparfait tense and an infinitive verb in French. When using two verbs in a sentence, the first verb is conjugated in the appropriate tense (in this case, the imparfait), while the second verb remains in the infinitive form.

However, there is one grammatical mistake in your sentence. The word "intelligente" should be changed to "intelligent" since the noun "peuple" is masculine, not feminine. So the correct sentence would be:

"Le peuple préhistorique était intelligent parce qu'il pouvait ériger les monuments sans machines."

To clarify how to construct such a sentence, start by conjugating the verb in the imparfait tense according to the subject (in this case, "le peuple" becomes "était" for the singular third person). After that, use the infinitive form of the second verb ("pouvoir" becomes "pouvait" in the imparfait tense). So, the structure is: subject + imparfait conjugation of the first verb + infinitive form of the second verb.

Remember to always double-check the agreement of adjectives with the noun they modify for gender and number.