Learn Turkish: Ayrıntıları görmek için neler sormak lazımdı? - In order to see the details, what all was it necessary to ask?

Neler is the plural form of the question word ne (what). We use it when we expect the answer to be a list of multiple things. By asking neler sormak lazımdı, the speaker is essentially asking "what are all the things that were necessary to ask?" rather than expecting just a single item in response.

Why does ayrıntılar take the accusative ending -ı (ayrıntıları) right before an infinitive?

Even though görmek (to see) is in its infinitive dictionary form here, it still acts like a verb and governs its own direct object. Because the sentence is referring to specific details ("the details"), that object needs the specific direct object marker, which is the accusative case -ı.

Why do we use the plain infinitive sormak with lazımdı here?

Using the plain infinitive sormak (to ask) with lazım expresses a general necessity, meaning "it was necessary to ask." If the speaker wanted to say a specific person needed to ask, they would use the short infinitive with a possessive ending, like sormam lazımdı (I needed to ask) or sorman lazımdı (you needed to ask).

Is lazımdı a past tense verb?

Not exactly! Lazım is an adjective meaning "necessary." To talk about the past, we attach the past tense copula (a "to be" suffix) -dı. So, lazımdı literally translates to "was necessary."