Этот мандарин кислее, чем вчерашний, а авокадо оказалось очень мягким.

Кислее is the comparative form of кислый (sour).

So:

• кислый = sour
• кислее = sourer / more sour

Russian often makes comparatives with endings like -ее or -ей. In this case:

• кислый → кислее

A native English speaker may want to translate it as more sour, and that is exactly the idea.

You can also say более кислый, but кислее is usually the more natural and compact choice in everyday Russian.

Чем is the normal word for than after a comparative.

So:

• кислее, чем вчерашний = sourer than yesterday’s one

This is the standard pattern:

• лучше, чем... = better than...
• хуже, чем... = worse than...
• больше, чем... = more than...

So here чем simply introduces the thing being compared.

Because the noun is understood from context and can be left out.

Here, вчерашний really means:

• вчерашний мандарин = yesterday’s tangerine/mandarin

But Russian often omits the noun when it is obvious. This is very common with adjectives that act like nouns in context.

So:

• Этот мандарин кислее, чем вчерашний
  literally: This mandarin is sourer than yesterday’s one

English does something similar with the red one, the old one, yesterday’s one.

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

Мандарин is:

• masculine
• singular
• nominative here (it is the subject)

So the correct form is:

• этот мандарин

Compare:

• эта книга = this book
• это окно = this window
• эти яблоки = these apples

Because авокадо is normally treated as a neuter noun in Russian.

Even though the word ends in -о and does not decline, it usually behaves grammatically like a neuter singular noun. So the past-tense verb must also be neuter:

• авокадо оказалось

Compare past tense agreement:

• он оказался = he / a masculine noun turned out to be
• она оказалась = she / a feminine noun turned out to be
• оно оказалось = it / a neuter noun turned out to be

So the form of the verb is chosen by grammar, not by the natural sex of something.

Because after оказаться (to turn out to be), the word describing what something turned out to be is very often put in the instrumental case.

So:

• авокадо оказалось очень мягким

literally means something like:

• the avocado turned out to be very soft

Here мягким is the instrumental form of мягкий.

This is a common pattern after verbs like:

• быть = to be
• стать = to become
• оказаться = to turn out to be

Especially with стать and оказаться, the instrumental is very typical.

Оказалось adds the idea of discovery or unexpected result:

• авокадо очень мягкое = the avocado is very soft
• авокадо оказалось очень мягким = the avocado turned out to be very soft

So the sentence is not just describing the avocado. It suggests that someone checked it, cut it open, touched it, or otherwise found out that it was very soft.

This makes the sentence feel more natural in a real-life context.

А often links two clauses with a mild contrast, comparison, or topic shift.

Here the sentence talks about two different fruits:

• Этот мандарин кислее, чем вчерашний, а авокадо оказалось очень мягким.

It is not a strong contradiction, so но would sound too strong.
It is not just simple addition either, so и is less precise.

А works well because it feels like:

• This mandarin is sourer than yesterday’s one, while/as for the avocado, it turned out very soft.

So а is very common when comparing or contrasting two separate things.

Yes, that is grammatically correct.

Both mean basically the same thing:

• Этот мандарин кислее, чем вчерашний
• Этот мандарин более кислый, чем вчерашний

But the first version with кислее is usually more natural and idiomatic in ordinary speech.

Russian often prefers the simple comparative form when it exists.

No, Russian word order is fairly flexible, because the endings carry a lot of grammatical information.

The given order is natural and neutral:

• Этот мандарин кислее, чем вчерашний, а авокадо оказалось очень мягким.

But other orders are possible for emphasis, for example:

• Авокадо оказалось очень мягким, а этот мандарин кислее, чем вчерашний.

That would put more focus on the avocado first.

Even so, the original version sounds very normal and is probably the best neutral choice for a learner to model.

The main stresses are:

• Э́тот
• мандари́н
• кисле́е
• чем
• вчера́шний
• а
• авока́до
• оказа́лось
• о́чень
• мя́гким

Stress is important in Russian, because it is not always predictable. A learner should especially notice:

• мандари́н
• кисле́е
• вчера́шний
• авока́до
• оказа́лось