Этот компот не такой сладкий, как тот, но мне он нравится.

Because этот is the masculine singular form of this, and компот is a masculine noun.

In Russian, words like this and that usually agree with the noun they describe in:

• gender
• number
• case

So:

• этот компот = this compote/drink
• эта книга = this book
• это окно = this window

Это can mean this/it/that is, but it is not the normal form used directly before a masculine noun here.

Russian nouns often show gender through their endings.

Компот ends in a consonant, and nouns with that pattern are very often masculine. So it takes masculine forms:

• этот компот
• сладкий компот
• он нравится

That is why both этот and сладкий are in masculine singular form.

Because Russian often omits a noun when it is already obvious from context.

So:

• как тот
  really means
• как тот компот

English does the same thing:

• This drink is not as sweet as that one

Russian simply leaves the noun out because the listener already knows what тот refers to.

This is a very common Russian comparison pattern:

• такой + adjective + как = as ... as
• with не, it becomes not as ... as

So:

• такой сладкий, как тот = as sweet as that one
• не такой сладкий, как тот = not as sweet as that one

This is one of the main ways Russian makes an equal-comparison.

Because как is used in this as ... as type of comparison.

Compare these two patterns:

1. такой/так + adjective/adverb + как
   = as ... as

   • не такой сладкий, как тот
   • not as sweet as that one

2. comparative + чем
   = more/less ... than

   • слаще, чем тот
   • sweeter than that one

So:

• не такой сладкий, как тот = not as sweet as that one
• менее сладкий, чем тот = less sweet than that one

Both are possible, but they are different comparison structures.

Because сладкий is the normal full adjective form agreeing with компот.

Here it is masculine singular nominative, matching компот:

• компот → masculine singular
• сладкий → masculine singular

Why not the others?

• сладко is usually an adverb or a predicative form, not the normal agreeing adjective here.
• сладок is a short-form adjective. It exists, but it sounds more literary or stylistically marked in many situations.

For everyday neutral Russian, сладкий is the natural choice.

Because нравиться works differently from English to like.

It literally works more like:

• it is pleasing to me

So in:

• мне он нравится

the parts are:

• мне = to me (dative)
• он = it/he (the thing that is pleasing)
• нравится = is pleasing / pleases

So Russian structures the idea differently from English.

That is why:

• Мне нравится компот = I like the compote
• but literally = The compote is pleasing to me

Because with нравиться, the person who feels the liking is put in the dative.

So:

• я → nominative = I
• мне → dative = to me

With нравиться, the grammar is:

• [thing liked] + нравится + [person in dative]

For example:

• Мне нравится чай. = I like tea.
• Ему нравится музыка. = He likes music.
• Нам нравится этот компот. = We like this compote.

So мне is not accidental; it is required by the verb.

Yes, it could be omitted:

• Этот компот не такой сладкий, как тот, но мне нравится.

That is understandable.

But он is often kept for clarity or emphasis, especially after a contrast like но.

So:

• но мне он нравится

feels like:

• but I do like it
• but as for me, I like it

The pronoun helps keep the focus on this compote as the thing being discussed.

Russian word order is flexible, and different orders give slightly different emphasis.

Both are possible:

• мне он нравится
• он мне нравится

The version in your sentence puts мне early, which can sound like:

• but I like it
• but to me, it is pleasing

That fits the contrast introduced by но.

So the chosen order is natural and slightly highlights the speaker’s personal reaction.

There are two different reasons.

1. Comma before как
   In this comparison structure, Russian normally separates the comparison part:

   • не такой сладкий, как тот

2. Comma before но
   Но joins two clauses, and Russian normally puts a comma before it:

   • ..., но мне он нравится

So the punctuation is standard for both the comparison and the contrasting second clause.

Yes, а is possible, but it gives a slightly different feel.

• но = but, with a clearer sense of contradiction or unexpectedness
• а = often and/but/whereas, softer contrast

So:

• ..., но мне он нравится
  suggests: it is not as sweet, but I still like it

• ..., а мне он нравится
  suggests more of a contrast in viewpoint: ..., whereas I like it

In this sentence, но works very well because it emphasizes that even though it is less sweet, the speaker likes it.

Learners often wonder about this. The most natural standard phrasing here is:

• не такой сладкий, как тот

That is the usual way to say not as sweet as that one with an adjective.

Using так instead of такой is much less natural in this exact structure. So if you want a safe, standard pattern, remember:

• такой + adjective + как
• не такой + adjective + как

For example:

• Он такой высокий, как брат.
• Этот чай не такой горячий, как тот.

So in your sentence, не такой сладкий is the form to learn and use.