Ben önemli cümlelerin altını çiziyorum.

Yes. In Turkish, the ending -um in çiziyorum already shows the subject is I.

So Ben önemli cümlelerin altını çiziyorum and Önemli cümlelerin altını çiziyorum both mean the same basic thing.

Including ben usually adds emphasis, such as:

• I am underlining the important sentences.
• As for me, I underline the important sentences.

So ben is optional here unless the speaker wants extra emphasis or contrast.

Önemli cümlelerin means of the important sentences.

Breakdown:

• önemli = important
• cümle = sentence
• cümleler = sentences
• cümlelerin = of the sentences

The ending -in / -ın / -un / -ün is the genitive ending, often corresponding to English of or possessive 's.

Here, Turkish uses a possessive structure:

• cümlelerin altı = the underside / lower part of the sentences

This is part of the fixed expression bir şeyin altını çizmek.

It is a very common Turkish expression meaning to underline.

Literally, it is something like:

• altı = its underside / the part under it
• çizmek = to draw

So bir şeyin altını çizmek is literally to draw its underside, which corresponds to to underline something.

It can also be used figuratively to mean to emphasize or to stress a point.

Because altını is the definite direct object form.

Here is the breakdown:

• altı = its underside / the lower part of it
• altını = its underside + accusative marker

In Turkish, definite direct objects usually take the accusative ending. Since the speaker is underlining a specific thing—the underside of those important sentences—Turkish uses altını.

So:

• cümlelerin altı = the underside of the sentences
• cümlelerin altını çiziyorum = I am underlining the sentences

This is a very natural question.

In Turkish possessive constructions, the possessed noun does not have to become plural just because the possessor is plural.

So:

• cümlelerin altı = the underside of the sentences

This is normal Turkish, especially in this fixed expression altını çizmek.

Even though there are multiple sentences, Turkish still commonly uses the singular possessed form altı in this idiomatic structure. It is best learned as the pattern:

• bir şeyin altını çizmek = to underline something

So with a plural noun:

• önemli cümlelerin altını çiziyorum

is the standard way to say it.

Çiziyorum is the present continuous form.

Breakdown:

• çiz- = draw
• -iyor- = present continuous
• -um = I

So çiziyorum means:

• I am drawing
• in this sentence, more naturally: I am underlining

Depending on context, Turkish -iyor can sometimes also correspond to English simple present:

• I underline important sentences
• I’m underlining important sentences

Both can be possible depending on the situation.

This is because of how the Turkish -iyor tense suffix works.

The present continuous suffix is basically -iyor, but the vowel before it can change depending on the verb stem:

• gelmek → geliyorum
• bakmak → bakıyorum
• görmek → görüyorum
• çizmek → çiziyorum

So from çiz-, you get çiziyor- + -um = çiziyorum.

This is a pattern you will see often, and it is best learned through examples rather than trying to force a simple one-rule spelling formula too early.

No. In Turkish, adjectives do not agree with nouns in number or case.

So:

• önemli cümle = important sentence
• önemli cümleler = important sentences
• önemli cümlelerin = of the important sentences

The adjective önemli stays the same throughout.

This is different from many European languages, where adjectives may change form.

The sentence follows the very common Turkish pattern:

• Subject + object + verb

So:

• Ben = subject
• önemli cümlelerin altını = object
• çiziyorum = verb

Turkish usually puts the verb at the end.

That said, Turkish word order is flexible, and changing the order can change the emphasis. But the version you have here is very natural and neutral.

It can be both.

Literally:

• underlining sentences in a book, article, or notes

Figuratively:

• emphasizing an idea or point

For this specific sentence, the most natural interpretation is probably literal, because it talks about sentences. But grammatically, the expression altını çizmek is also widely used in figurative contexts, especially in formal speech and writing.