Learn Hardcore Icelandic: Ég tek pennann úr vasanum.

Ég tek pennann... = I take the pen... (habitual or immediate action)
It can also be used for near-future/planned actions depending on context (similar to English I’m taking... / I take... in schedules).
If you want to be very explicit about future, Icelandic can use adverbs (like á eftir) or phrasing that makes the future meaning clear, but plain present is very common.

Why is pennann in that form? What case is it?

Pennann is accusative singular with the definite article attached, because taka takes a direct object in the accusative.

Base noun: penni (a pen)
Accusative singular (indefinite): penna
Accusative singular (definite): pennann = the pen

So pennann literally means the pen (as the thing being taken).

How do I say a pen instead of the pen here?

Use the indefinite accusative:

Ég tek penna úr vasanum. = I take a pen out of the pocket.

Definiteness in Icelandic is usually shown by a suffix (like -inn/-in/-ið) rather than a separate word like the.

Why is it úr vasanum and not something like úr vasi?

The preposition úr (out of/from) governs the dative case, so vasi must appear in the dative.

vasi (nominative singular) = pocket
vasa (accusative/genitive singular)
vasa (dative singular, indefinite)
vasanum (dative singular, definite) = the pocket

So úr vasanum means out of the pocket (a specific pocket).

Why does vasanum end in -um?

Because it’s dative singular definite of a masculine noun (vasi).
A very common pattern is:

dative singular definite masculine: -num or -inum/-anum/-unum depending on the stem
Here it’s vasi → vasanum.

Think of -num as basically the + dative bundled together.

What’s the difference between úr vasa and úr vasanum?

It’s definiteness:

úr vasa = out of a pocket (some pocket, not specified)
úr vasanum = out of the pocket (a specific one, implied to be known from context)

Both are grammatically correct; the choice is about whether the pocket is treated as definite.

Is úr the only option? Could I use af instead?

Usually you use úr for movement from inside something:

úr vasanum = out of the pocket (inside → outside)

Af is more like off/from the surface of something:

taka af borðinu = take (it) off the table

So for a pocket, úr is the natural choice.

Why is the word order Ég tek pennann úr vasanum?

This is a very typical Icelandic order: 1) subject (Ég)
2) verb (tek)
3) object (pennann)
4) prepositional phrase (úr vasanum)

Icelandic word order is flexible, but this is the neutral, straightforward pattern for a simple statement.

Can I drop Ég like in Spanish or Italian?

Not usually. Icelandic is not generally a pro-drop language, so the subject pronoun is normally included:

Ég tek pennann... is the default.

In some contexts (especially answers, diary-style notes, or coordinated clauses) the subject can be omitted, but learners should treat Ég as required in normal sentences.

How do I add my pocket: out of my pocket?

Add the possessive adjective, and it agrees in case/gender/number with vasi (masculine, dative singular):

Ég tek pennann úr vasanum mínum. = I take the pen out of my pocket.

(You can also sometimes see possessives placed before the noun, but the postposed form like vasanum mínum is very common.)

How do you pronounce this sentence, especially Ég, tek, pennann, úr?

A practical learner-oriented guide:

Ég: sounds roughly like yeh(g); the g is often very soft or disappears depending on speaker and position.
tek: like tehk (short e).
pennann: roughly PEHN-nahn; the double nn signals a longer/stronger n sound.
úr: like oor (long ú).
vasanum: roughly VAH-sa-num.

Exact pronunciation varies by region, but those approximations will usually be understood.