Onuncu soru zordu.

Onuncu means tenth.

It is formed from on (ten) plus the ordinal suffix -(I)ncI, which makes ordinal numbers in Turkish:

• bir → birinci = first
• iki → ikinci = second
• on → onuncu = tenth

So onuncu soru means the tenth question.

This is because of vowel harmony, a very important feature of Turkish.

The ordinal suffix changes shape to match the vowels in the word it attaches to. After on, the correct form is -uncu, giving onuncu.

You do not need to memorize a separate rule just for ten; it is part of the normal vowel-harmony pattern seen in ordinal numbers:

• altı → altıncı
• dokuz → dokuzuncu
• on → onuncu

Because in Turkish, adjectives and adjective-like words normally come before the noun they describe.

So:

• onuncu soru = the tenth question
• zor soru = a difficult question

Here, onuncu is describing soru, so it comes first.

Turkish does not have a direct equivalent of the English definite article the.

So soru can mean:

• question
• the question
• sometimes a question

depending on context.

In English, ordinal numbers usually take the, so onuncu soru is naturally translated as the tenth question. Turkish does not need a separate word for that.

Because zordu means was difficult, while zor by itself means is difficult (or just difficult, depending on context).

Compare:

• Onuncu soru zor. = The tenth question is difficult.
• Onuncu soru zordu. = The tenth question was difficult.

So the -du part marks past time here.

It is built into zordu.

Turkish often does not use a separate word like English was in sentences like this. Instead, a past-tense ending is added directly to the predicate:

• zor = difficult
• zor-du = was difficult

So zordu is basically difficult-was.

-du is the past tense marker used with this kind of predicate.

In this sentence:

• zor = difficult
• zordu = was difficult

This ending can appear in different forms because of vowel harmony and consonant harmony:

• -dı
• -di
• -du
• -dü
• and sometimes -tı, -ti, -tu, -tü

Here it is -du because that is the form that fits zor.

In the third person singular, Turkish often has no extra personal ending in this structure.

So:

• zordum = I was difficult / I was hard
• zordun = you were difficult / you were hard
• zordu = he/she/it was difficult / hard

In your sentence, the subject is onuncu soru, so zordu is enough by itself.

Yes. This is the most neutral and natural word order.

A basic Turkish sentence often puts the predicate at the end, so:

• Onuncu soru = the tenth question
• zordu = was difficult

Together: Onuncu soru zordu.

Other word orders are possible in Turkish, but they usually add emphasis or sound more marked. The version you have is the standard one.

The dictionary form is zor.

Zor is an adjective meaning difficult / hard.
When you add the past marker, it becomes zordu = was difficult.

So if you look it up in a dictionary, you would search for zor, not zordu.