Ich mag Quark lieber als Joghurt, besonders wenn er cremig ist.

Mag is the 1st person singular form of mögen in the present tense:

• ich mag = I like
• du magst
• er/sie/es mag
• wir mögen
• ihr mögt
• sie/Sie mögen

So Ich mag Quark ... simply means I like quark ...

German often uses mögen for general likes and preferences, especially with nouns.

Lieber ... als ... is the normal way to say prefer ... to ... with verbs like mögen.

So:

• Ich mag Quark lieber als Joghurt. = I like quark better than yogurt / I prefer quark to yogurt.

A useful pattern is:

• Ich mag A lieber als B.

This is very common in everyday German.

Usually, lieber is the better and more natural choice here.

• lieber expresses preference
• mehr usually expresses more in the sense of a greater amount or sometimes can create ambiguity

So:

• Ich mag Quark lieber als Joghurt. = I prefer quark to yogurt.
• Ich mag Quark mehr als Joghurt. can sound less natural for simple preference, and it may even be understood as I like quark more than yogurt does in some contexts.

For a clear comparison of preference, lieber is the standard word.

In German, als is the normal word used for comparisons of inequality, like than in English.

Examples:

• größer als = bigger than
• besser als = better than
• lieber als = rather than / more gladly than

So in this sentence:

• Quark lieber als Joghurt = quark rather than yogurt

By contrast, wie is used for equality:

• so groß wie = as big as

German often leaves out the article when talking about foods in a general sense, especially after verbs like mögen, essen, trinken, and so on.

So these are natural:

• Ich mag Quark.
• Ich esse Joghurt.
• Sie trinkt Kaffee.

This is similar to English when we say I like coffee rather than I like the coffee, unless we mean a specific coffee.

If you were talking about a specific one, you could use an article:

• Ich mag den Joghurt im Kühlschrank. = I like the yogurt in the fridge.

Er is the pronoun that refers back to a masculine noun.

Both der Quark and der Joghurt are masculine nouns in German, so the pronoun for either one is er.

In this sentence, the most natural interpretation is that er refers to Quark, because Quark is the main thing being talked about:

• Ich mag Quark lieber als Joghurt, besonders wenn er cremig ist.

So the idea is:

• I prefer quark to yogurt, especially when the quark is creamy.

Because both nouns are masculine, er could theoretically be ambiguous, but in context most people understand it as referring to Quark.

Because wenn introduces a subordinate clause, and in German subordinate clauses send the conjugated verb to the end.

So:

• main clause: Er ist cremig.
• subordinate clause: wenn er cremig ist

This is a very important German word-order rule.

Other examples:

• weil er müde ist = because he is tired
• dass sie kommt = that she is coming
• wenn ich Zeit habe = if/when I have time

Because cremig is a predicate adjective here, not an adjective directly before a noun.

Compare:

• der cremige Quark = the creamy quark
  → adjective before a noun, so it gets an ending

• Der Quark ist cremig. = the quark is creamy
  → adjective after sein, so it does not get an ending

In your sentence:

• wenn er cremig ist = when it is creamy

So cremig stays in its basic form.

Besonders means especially.

It adds extra emphasis:

• Ich mag Quark lieber als Joghurt = I prefer quark to yogurt.
• ..., besonders wenn er cremig ist = ..., especially when it is creamy.

So it tells us that the speaker likes quark even more under that condition.

Yes, German adverbs are often somewhat flexible, but the original position is very natural.

For example:

• Ich mag Quark lieber als Joghurt, besonders wenn er cremig ist.
• Besonders mag ich Quark, wenn er cremig ist.

These do not sound exactly the same in emphasis, but both are possible.

The original version is a good, neutral way to say it.

In this sentence, when is usually the best match.

• besonders wenn er cremig ist = especially when it is creamy

Here, the meaning is a general condition or recurring situation: whenever the quark is creamy, the speaker likes it especially much.

German wenn can often mean:

• when for repeated or general situations
• if for conditions

So context decides the best English translation.

Yes. The usual noun is der Quark, so it is masculine.

That is why the pronoun is er:

• der Quark → er

Likewise:

• der Joghurt → er

This is grammatical gender, so it does not mean the food is biologically male; it is just part of the noun’s grammar.

Yes, it sounds very natural.

It uses several common everyday structures:

• Ich mag ...
• lieber als ...
• besonders wenn ...

So it is a good example of normal spoken and written German.