Learn Hardcore Spanish (Latin America): Subrayo la línea importante en mi libro verde con mi bolígrafo nuevo. - I underline the important line in my green book with my new pen.

In Spanish the default position for descriptive adjectives is after the noun: línea importante. Putting it before—importante línea—is possible but gives a more literary or poetic feel. Most everyday speech and writing place adjectives after the noun.

How do you conjugate subrayar in the present tense for yo?

Subrayar is a regular –ar verb. In the present tense:
yo subrayo
tú subrayas
él/ella subraya
nosotros subrayamos
vosotros subrayáis
ellos subrayan

Why do we say en mi libro verde instead of de mi libro verde?

The preposition en indicates location: you underline in the book.
Using de would mean “of my green book” and change the meaning: e.g. la tapa de mi libro = “the cover of my book.”

Why is it bolígrafo nuevo and not bolígrafo nueva?

Adjectives in Spanish agree in gender and number with the noun they modify. Bolígrafo is masculine singular, so the adjective is nuevo (masculine singular), not nueva (feminine).

Can we say Subrayo la línea importante de mi libro verde? What changes?

Yes, you can. de mi libro verde (“of my green book”) still works: you’re specifying “the important line of my green book.” The nuance:

en mi libro verde focuses on the place where you underline.
de mi libro verde emphasizes the line belonging to that book.

Why use con to express “with my new pen”? Are there alternatives?

Con + instrument is the usual way to say “with [tool]”: con mi bolígrafo nuevo.
Alternative: uso mi bolígrafo nuevo (“I use my new pen”), but that changes the verb: you’re now focusing on using the pen, not merely stating that it’s the instrument for underlining.

What’s the difference between subrayar and resaltar?

Subrayar: literally “to underline” (draw a line under text).
Resaltar: “to highlight,” “to emphasize.” You can resaltar ideas verbally or in writing (often with color), not necessarily with an underline.

Why use the simple present subrayo instead of the present progressive estoy subrayando?

In Spanish, the simple present often describes current actions.

Subrayo la línea importante = “I underline the important line” (can be habitual or happening now).
Estoy subrayando la línea importante emphasizes that the action is in progress at this moment.

Can you put the possessive mi after the noun, like libro mi?

No. In Spanish, possessive adjectives (mi, tu, su, etc.) go before the noun: mi libro. Placing them after is ungrammatical, except in some poetic expressions (rare).

Can you omit the article and say Subrayo línea importante en mi libro verde?

No. Spanish usually requires the definite article before singular, specific nouns: la línea importante. Omitting la sounds unnatural and ungrammatical in this context.