Jo subratllo cada paràgraf important amb un retolador blau.

You can usually leave it out.

Catalan often drops subject pronouns because the verb ending already shows who is doing the action.
Here, subratllo means I underline / I highlight, so jo is not strictly necessary.

• Jo subratllo cada paràgraf important... = I underline each important paragraph...
• Subratllo cada paràgraf important... = same basic meaning

Using jo adds emphasis, contrast, or clarity, for example:

• Jo subratllo, però tu no. = I underline, but you don’t.

Subratllo is the 1st person singular present form of subratllar, which means to underline or sometimes to highlight/mark as important, depending on context.

So:

• subratllar = to underline
• subratllo = I underline

In this sentence, it describes a habitual action or something the speaker generally does.

Because subratllo is the form for I, while subratlla is the form for he/she/it or you formal.

Present tense of subratllar:

• jo subratllo = I underline
• tu subratlles = you underline
• ell/ella/vostè subratlla = he/she/you formal underlines
• nosaltres subratllem
• vosaltres subratlleu
• ells/elles/vostès subratllen

So the -o ending here is the normal I ending for many -ar verbs in Catalan.

A useful approximate pronunciation for an English speaker is soo-brat-lyo.

A few notes:

• sub- sounds roughly like soob
• the stress falls on rat
• ll in Catalan is traditionally a palatal sound, roughly similar to a soft ly sound

So subratllo is approximately soo-BRAT-lyo.

You do not pronounce it like English sub-rat-low.

Because cada means each, and in Catalan it is followed by a singular noun.

So:

• cada paràgraf = each paragraph

This works just like:

• cada dia = each day
• cada llibre = each book

English does the same: we say each paragraph, not each paragraphs.

Because in Catalan, adjectives usually come after the noun.

So:

• paràgraf important = important paragraph
• retolador blau = blue marker

This is one of the most common differences from English word order.

Catalan often uses:

• noun + adjective

rather than:

• adjective + noun

There are exceptions, but in this sentence the normal order is used.

For the same reason: blau is an adjective, and Catalan usually puts adjectives after the noun they describe.

So:

• un retolador blau = a blue marker

Here:

• retolador = marker
• blau = blue

This is the normal Catalan order.

Because retolador is a masculine singular noun, so the adjective must match it.

• retolador = masculine singular
• blau = masculine singular form of blue

Compare:

• un retolador blau = a blue marker
• una carpeta blava = a blue folder

So the adjective agrees with the noun in gender and number.

Amb means with.

In this sentence, it introduces the tool used to do the action:

• amb un retolador blau = with a blue marker

So the structure is:

• subratllo ... amb un retolador blau
• I underline ... with a blue marker

Retolador usually means marker pen, felt-tip pen, or highlighter/marker, depending on context.

In this sentence, it refers to the writing tool used to underline. A learner should understand it broadly as marker.

Depending on the exact object, Catalan speakers might also use more specific words in other contexts, but retolador is a very natural general word here.

Important is an adjective describing paràgraf.

So:

• cada paràgraf important = each important paragraph

It tells you which paragraphs are being underlined: not every paragraph, only the important ones.

Also note that important looks almost identical to English important, which makes it easy to recognize, even though the pronunciation is different.

Yes. Subratllo is in the present tense.

In Catalan, the present tense can mean:

• something happening now
• something someone usually does
• a general habit

So this sentence can naturally mean:

• I underline each important paragraph with a blue marker
• or I usually underline each important paragraph with a blue marker

The exact interpretation depends on context.

Yes, Catalan allows some flexibility in word order, but the original sentence is the most neutral and natural.

The standard version is:

• Jo subratllo cada paràgraf important amb un retolador blau.

You might also hear variations for emphasis, but beginners should learn the basic order first:

• subject + verb + object + phrase with amb

So this sentence is a very good model for normal word order.