i japjineun gwanggoneun jeokjiman ilgeul manhan gisaga manha.

는 marks the topic, not just the subject.

So 이 잡지는 means something like:

• As for this magazine, ...
• This magazine, ...

It sets this magazine as the thing being talked about. In English, we often do not mark topics this clearly, but Korean does.

If you said 이 잡지가, that would sound more like you are specifically identifying this magazine as the grammatical subject in a more neutral or contrast-free way. With 는, the sentence feels more like a comment about the magazine as a whole.

The second 는 is doing a contrastive job.

• 이 잡지는 = As for this magazine, ...
• 광고는 적지만 = the ads, though, are few / as for ads, they are few, but...

Here, 는 on 광고 highlights one aspect of the magazine and sets up a contrast with what comes next:

• 광고는 적지만 = ads are few, but...
• 읽을 만한 기사가 많아 = there are many worthwhile articles

So the sentence is contrasting two features of the magazine:

• fewer ads
• many good articles

This kind of repeated 는 is very common in Korean.

적지만 comes from:

• 적다 = to be few / to be little in number
• -지만 = but / although

So:

• 적다 → 적지만
• literally: it is few, but...
• naturally: there are few ..., but ...

In this sentence:

• 광고는 적지만 = there are few ads, but...

Even though 적다 is an adjective, in English we often translate it with there are few.

It can feel like either, depending on how you translate it.

-지만 connects two clauses and shows contrast. So it can mean:

• but
• although / even though

In this sentence, the most natural English is probably:

• There are few ads, but there are many articles worth reading.

But the nuance of although is also present:

• Although it has few ads, it has many worthwhile articles.

So -지만 is a general contrast connector.

읽을 만한 means worth reading or good enough to read.

It comes from the pattern:

• verb stem + -(으)ㄹ 만하다

This pattern means:

• to be worth doing
• to be reasonable/satisfactory enough to do

So:

• 읽다 = to read
• 읽을 만하다 = to be worth reading

Then 읽을 만한 기사 means:

• an article worth reading
• a readable / worthwhile article

In the sentence:

• 읽을 만한 기사가 많아 = there are many articles worth reading

Because Korean uses a noun-modifying form before a noun.

Here is the structure:

• 읽다 = to read
• 읽을 만하다 = to be worth reading
• 읽을 만한 기사 = an article that is worth reading

The 한 in 만한 is the adjective form that modifies the noun 기사.

So the whole chunk 읽을 만한 works like an English adjective phrase before 기사.

You can think of it like this:

• 읽을 만하다 = is worth reading
• 읽을 만한 기사 = a worth-reading article / an article worth reading

Because 많다 often takes the thing that is numerous with 가/이.

So:

• 기사가 많아 = there are many articles
• literally: articles are many

This is a very common Korean pattern:

• 사람이 많다 = there are many people
• 문제가 많다 = there are many problems
• 기사가 많다 = there are many articles

Even though English often uses there are, Korean often uses a noun + 가/이 + adjective like 많다 or 적다.

Yes, literally it is closer to ads are few.

Korean often expresses quantity with adjectives like:

• 많다 = to be many
• 적다 = to be few

So instead of saying there are few ads, Korean can simply say:

• 광고가 적다
• literally: ads are few
• naturally: there are few ads

In this sentence, because of the contrastive 는, it becomes:

• 광고는 적지만
• as for ads, they are few, but...

So the Korean grammar is different from English, but the meaning is the same.

많아 is the casual/informal polite-neutral style, often called 반말.

Compare:

• 많다 = dictionary/plain form
• 많아요 = polite
• 많아 = casual

So this sentence sounds like something you would say to:

• a friend
• someone younger
• in casual writing/dialogue

If you wanted a polite version, it would be:

• 이 잡지는 광고는 적지만 읽을 만한 기사가 많아요.

It is not a single fixed phrase, but it uses a very common and productive pattern.

The pattern is:

• -(으)ㄹ 만하다 = worth doing / good enough to do

So you can make many expressions like:

• 볼 만하다 = worth seeing
• 먹을 만하다 = worth eating / decent enough to eat
• 살 만하다 = worth living / livable, depending on context
• 들을 만하다 = worth listening to

So 읽을 만한 기사 is a normal, natural combination meaning articles worth reading.

Because Korean puts descriptive material before the noun.

English can say:

• There are many articles worth reading.
• Many articles are worth reading.

Korean prefers:

• 읽을 만한 기사 = articles worth reading
• then adds 가 많아 = there are many

So the Korean structure is:

• 읽을 만한 = worth reading
• 기사 = articles
• 가 많아 = are many / there are many

A natural way to understand it is:

• There are many articles that are worth reading.

This is very typical Korean word order.