Этот ананас оказался слаще, чем я ожидал.

Оказался means turned out to be or proved to be. It suggests that the result was discovered, and often that it was a little surprising.

So:

• Этот ананас был слаще... = This pineapple was sweeter...
• Этот ананас оказался слаще... = This pineapple turned out to be sweeter...

In this sentence, оказался is more natural because the speaker is comparing the real taste with their earlier expectation.

Because оказался is past tense, masculine singular, and it agrees with ананас.

• ананас = masculine noun
• so the verb in the past tense is masculine too: оказался

Compare:

• Этот ананас оказался... = masculine
• Эта груша оказалась... = feminine
• Это яблоко оказалось... = neuter

In Russian past tense, verbs agree in gender and number with the subject.

Слаще is the comparative form of сладкий (sweet). It means sweeter.

So:

• сладкий = sweet
• слаще = sweeter

Russian often forms comparatives with a special short form like this, instead of using a separate word like English more.

Other examples:

• быстрый → быстрее = faster
• дорогой → дороже = more expensive
• сладкий → слаще = sweeter

You can sometimes also say более сладкий, but слаще is more natural here.

Чем means than in comparisons.

So:

• слаще, чем я ожидал = sweeter than I expected

This is the standard way to make this kind of comparison in Russian.

A simple pattern is:

• X больше, чем Y = X is bigger than Y
• X лучше, чем Y = X is better than Y
• X слаще, чем Y = X is sweeter than Y

Ожидал is past tense: expected.

The sentence compares the pineapple’s actual taste with what the speaker expected earlier, so past tense makes sense:

• я ожидал = I expected

If you said я ожидаю, that would mean I expect / am expecting now, which does not fit the meaning as well here.

Yes. In Russian, past tense verbs are usually formed with -л plus gender/number endings.

For ожидать:

• ожидал = expected, masculine speaker
• ожидала = expected, feminine speaker
• ожидало = expected, neuter
• ожидали = expected, plural

So я ожидал tells us the speaker is male.
If the speaker were female, it would be:

• Этот ананас оказался слаще, чем я ожидала.

This is something English speakers often notice, because English past tense does not show the speaker’s gender.

Russian often leaves out words that are understood from context.

Here, я ожидал really means something like:

• than I expected
• more fully: than I expected it to be

But Russian does not need to say the object explicitly if it is obvious. The listener understands that the speaker means the pineapple’s sweetness or the pineapple itself.

English does something similar in This is better than I expected.

Ананас is in the nominative case because it is the subject of the sentence.

• Этот ананас = this pineapple
• it is the thing that turned out to be sweeter

So:

• этот is masculine nominative singular
• ананас is masculine nominative singular

They match because этот describes ананас.

Этот means this and must agree with ананас in gender, number, and case.

Since ананас is:

• masculine
• singular
• nominative

the correct form is:

• этот

Compare:

• этот ананас = this pineapple
• эта груша = this pear
• это яблоко = this apple
• эти ананасы = these pineapples

Yes, grammatically you could say that, but слаще is more natural and more concise here.

Compare:

• Этот ананас оказался слаще, чем я ожидал.
• Этот ананас оказался более сладким, чем я ожидал.

The second version is possible, but notice that with более сладким the grammar changes because it behaves more like an adjective phrase after оказался. The simple comparative слаще is the more straightforward and common choice in this sentence.

Yes, in this sentence the comma is normally used before чем.

That is because чем я ожидал is treated as a comparative clause:

• Этот ананас оказался слаще, чем я ожидал.

Russian punctuation often uses a comma before чем when it introduces a full comparison like than I expected.

The stress is:

• оказа́лся
• сла́ще

So the sentence is pronounced approximately like:

• Э́тот анана́с оказа́лся сла́ще, чем я ожида́л.

Stress is important in Russian, especially for learners, because it is not always predictable from spelling.