To pytanie było najtrudniejsze na egzaminie.

Because pytanie is a neuter noun in Polish. Demonstratives like ten / ta / to must agree with the noun’s gender.

• ten = masculine
• ta = feminine
• to = neuter

So:

• to pytanie = this question

Even though question is not neuter in English, Polish nouns have their own grammatical gender, and pytanie happens to be neuter.

Here, to is a demonstrative adjective/pronoun meaning this (sometimes that, depending on context).

So to pytanie usually means:

• this question
• sometimes that question, if the context makes that clearer

Polish to is less rigid than English this/that, so context matters more.

Yes, there is a useful pattern. Many Polish nouns ending in -anie, -enie, or -cie are neuter.

So:

• pytanie = question
• zadanie = task
• czytanie = reading

That does not mean every noun with a similar ending works the same way, but it is a very common pattern.

Było is the past tense of być (to be) in the neuter singular form.

Past tense forms of być change for gender in the singular:

• był = masculine
• była = feminine
• było = neuter

Since pytanie is neuter singular, the sentence needs było:

• To pytanie było...

Because the adjective must agree with pytanie in gender, number, and case.

The adjective comes from:

• trudny = difficult
• trudniejszy / trudniejsze = more difficult
• najtrudniejszy / najtrudniejsze = most difficult

Since pytanie is neuter singular, the correct form is:

• najtrudniejsze

Compare:

• ten test był najtrudniejszy — masculine
• ta część była najtrudniejsza — feminine
• to pytanie było najtrudniejsze — neuter

Polish usually forms the superlative by adding naj- to the comparative form.

So:

• trudny = difficult
• trudniejszy / trudniejsze = more difficult
• najtrudniejszy / najtrudniejsze = most difficult

So najtrudniejsze literally corresponds to the most difficult.

Because after być with an adjective, Polish normally uses the nominative, not the instrumental.

So:

• To pytanie było najtrudniejsze. — correct

The instrumental is much more typical when the complement is a noun, for example:

• On był nauczycielem. = He was a teacher.

But with adjectives describing what something was like, nominative is standard:

• Dom był duży.
• Pytanie było trudne.
• To pytanie było najtrudniejsze.

Because Polish uses na with certain events, activities, and contexts where English often uses in or on.

So:

• na egzaminie = on the exam / in the exam context

Also, after na in a location/context meaning, the noun usually goes into the locative case. That is why:

• egzamin → egzaminie

Because w egzaminie is not the normal Polish expression here.

Polish idiomatically says:

• na egzaminie = on the exam / during the exam

This is just one of those preposition patterns that learners need to get used to. English and Polish do not always divide up in / on / at the same way.

It is the locative singular form of egzamin.

You have:

• nominative: egzamin
• locative: egzaminie

It appears here because na + location/context uses the locative:

• na egzaminie
• na stole
• na lekcji

Yes, that is grammatical.

• To pytanie było najtrudniejsze na egzaminie.
• Pytanie było najtrudniejsze na egzaminie.

The version with to sounds more like you are pointing out a specific question:

• this question
• that question

Without to, the noun phrase is less explicitly demonstrative and can sound a bit more general or less pointed.

No, Polish word order is fairly flexible, although the original sentence is a very natural neutral order.

You could also hear:

• Najtrudniejsze na egzaminie było to pytanie.
• Na egzaminie to pytanie było najtrudniejsze.

These alternatives change the focus or emphasis, not the basic meaning.

The original:

• To pytanie było najtrudniejsze na egzaminie.

is a straightforward way to say it.

It can mean either, depending on how you translate naturally into English.

• najtrudniejsze = the hardest
• najtrudniejsze = the most difficult

Both are good English translations. Polish often uses one form where English may choose either a shorter everyday version (hardest) or a more formal one (most difficult).