Karpuzu kesmeden önce armutları yıkadım, çünkü çocuklar ikisini de seviyor.

They are marked because they are definite direct objects.

• karpuz → karpuzu
• armutlar → armutları

In Turkish, a specific/directly affected object often takes the accusative ending. So here the speaker means something like:

• the watermelon
• the pears

rather than just some watermelon or some pears in a vague sense.

This is one of the most important uses of the Turkish accusative.

kesmeden önce means before cutting.

It is built like this:

• kes- = cut
• -meden = without doing / before doing in this pattern
• önce = before

So:

• kesmeden önce = before cutting
• gitmeden önce = before going
• yemeden önce = before eating

Even though -meden/-madan by itself often means without doing, when it is followed by önce, the whole expression means before doing.

Because Turkish usually uses postpositions where English uses prepositions.

English says:

• before cutting

Turkish says, more literally:

• cutting before

So önce comes after the word or phrase it relates to:

• yemekten sonra = after eating
• gelmeden önce = before coming

This is very normal Turkish structure.

yıkadım breaks down as:

• yıka- = wash
• -dı = past tense
• -m = I

So yıkadım means I washed.

You will often see this pattern:

• geldim = I came
• yaptım = I did
• buldum = I found
• yıkadım = I washed

The exact vowel/consonant shape of the tense ending changes because of vowel harmony and consonant harmony.

In Turkish, the -iyor form is used not only for actions happening right now, but also very often for present states and current general facts.

So seviyor can naturally mean:

• likes
• is fond of
• loves

In this sentence, çocuklar ikisini de seviyor means the children like both.

With verbs like sevmek, Turkish very commonly uses -iyor for a present meaning that English usually expresses with the simple present.

Because in Turkish, when the plural subject is already clearly stated, the verb is often left in the third-person singular form.

So these are both possible:

• Çocuklar ikisini de seviyor.
• Çocuklar ikisini de seviyorlar.

The first one is very common and natural. The second one is also possible, but often feels a bit more explicit or emphatic.

So this is not a mistake: Turkish does not always match plural subjects with a plural verb the way English does.

ikisini de means both of them.

It refers back to the two things just mentioned:

• karpuzu = the watermelon
• armutları = the pears

Here is the breakdown:

• ikisi = the two of them
• ikisini = the two of them, as an object
• de = adds the sense of also / too / both

So:

• ikisini de seviyor = likes both of them

This is a very common Turkish pattern:

• birini = one of them
• ikisini = the two of them / both of them
• üçünü = the three of them

Because ikisini is the pronoun-like form meaning the two of them, while ikiyi is just the number two in the accusative.

Compare:

• ikiyi = the number two
• ikisini = both of them / the two of them

Since the sentence refers back to two previously mentioned things, Turkish uses ikisini.

So ikisini de seviyor is the natural way to say likes both of them.

Yes. Out of context, armutları can be ambiguous.

It can mean:

• the pears
• his/her pears
• sometimes their pears, depending on context

In this sentence, the most natural reading is simply the pears. But Turkish often leaves this kind of thing to context.

That is very normal in Turkish, and learners quickly get used to it.

Because Turkish does not have a separate word exactly like English the.

Instead, definiteness is often shown by:

• context
• word order
• especially the accusative ending on direct objects

So karpuzu and armutları feel definite partly because they are accusative objects.

That is why Turkish can express the watermelon and the pears without using a separate article.

No, Turkish word order is flexible, but the original sentence has a very natural neutral order.

Original:

• Karpuzu kesmeden önce armutları yıkadım, çünkü çocuklar ikisini de seviyor.

This structure sounds normal because it goes:

1. time/background phrase
2. main object
3. main verb
4. reason clause

You can move parts around for emphasis, but that changes the focus. For example:

• Armutları, karpuzu kesmeden önce yıkadım.
  This emphasizes the pears.

So the sentence is not rigidly fixed, but the given order is a good standard one.

çünkü is a very common way to introduce a reason:

• ..., çünkü ... = ..., because ...

So:

• çünkü çocuklar ikisini de seviyor = because the children like both

There are other Turkish ways to express because, such as -dığı için, but çünkü is straightforward and very common in everyday language.

For example, a different version might be:

• Çocuklar ikisini de sevdiği için, karpuzu kesmeden önce armutları yıkadım.

That also means roughly the same thing, but the structure is different.

In this sentence, it most naturally refers to the two things mentioned in the discourse:

1. the watermelon
2. the pears

So it does not have to mean exactly two physical fruits. It can mean both of those things as two mentioned items or categories.

That is why the sentence works even though one noun is singular and the other is plural. Turkish is grouping them as two referents in the conversation.