Ég geri þetta fyrir þig.

Because gera is the infinitive, meaning to do / to make.

In Ég geri þetta fyrir þig, the verb is conjugated for 1st person singular present tense:

• ég geri = I do / I am doing
• að gera = to do

This is an irregular present-tense form, so you have to learn it as part of the verb’s pattern.

A few present-tense forms of gera are:

• ég geri = I do
• þú gerir = you do
• hann/hún/það gerir = he/she/it does

Here is the role of each word:

• Ég = subject, I
• geri = verb, do / am doing
• þetta = direct object, this
• fyrir þig = prepositional phrase, for you

So the structure is:

subject + verb + object + prepositional phrase

That is a very common Icelandic sentence pattern.

Þetta is the neuter singular form of þessi (this).

Here it is being used as the direct object of the verb gera, so it appears in the form þetta.

You can think of it as:

• þetta = this, referring to a thing, situation, or action

In Icelandic, demonstratives change form depending on gender, number, and case.
So þetta is not the only form of this—it is just the one that fits here.

Because þig is the accusative form of þú (you).

The preposition fyrir often takes the accusative when it means for, especially in the sense of doing something for someone’s benefit. So:

• þú = you (subject form / nominative)
• þig = you (accusative form)

That is why Icelandic says:

• fyrir þig = for you

not fyrir þú.

Here fyrir takes the accusative.

In Icelandic, some prepositions can govern different cases depending on meaning. With fyrir, one very common meaning is for someone / on someone’s behalf / for someone’s benefit, and in that use it normally takes the accusative.

So in this sentence:

• fyrir þig = for you
• þig is accusative

This is something learners need to get used to: with prepositions, you often have to learn both the meaning and the case they require.

It can mean either, depending on context.

Icelandic present tense often covers both:

• I do this for you
• I am doing this for you

Unlike English, Icelandic does not usually need a separate progressive form like am doing. The situation tells you whether it is a general statement or something happening right now.

The sentence as written has the neutral word order:

• Ég geri þetta fyrir þig.

But Icelandic word order can shift for emphasis. For example:

• Þetta geri ég fyrir þig.

That still means basically the same thing, but now þetta is emphasized: This I do for you.

Icelandic is a verb-second language in main clauses, so if something other than the subject comes first, the finite verb usually still stays in the second position.

Usually, no.

In Icelandic, subject pronouns are generally expressed much more often than in languages like Spanish or Italian. So:

• Ég geri þetta fyrir þig is the normal full sentence.

If you just said Geri þetta fyrir þig, it could sound incomplete in many contexts, although in informal speech or conversation some omission can happen when the subject is obvious. For standard learning purposes, keep ég.

It can refer to more than a physical object.

Þetta is often used for:

• a thing: this thing
• an action: this
• a situation or matter: this situation / this matter

So in Ég geri þetta fyrir þig, þetta could mean:

• this task
• this favor
• this action
• this thing I’m working on

The exact reference depends on context.

A rough learner-friendly pronunciation is:

yeg GEH-rɪ THET-ta FIH-rir thig

A few important notes:

• Ég begins with a sound somewhat like yehg
• g in geri is not always a hard English g
• þ is like English th in thin
• ð is like English th in this, though in some positions it can be weaker
• þig is often pronounced with a soft th sound at the start

Exact pronunciation varies a bit by speaker, but those are the main sounds to notice.

Not always.

Gera is very common and can mean do or make, depending on context. For example, it can be used for:

• doing an action
• making something
• causing something
• carrying something out

So in this sentence, English happens to use do, but in other sentences gera might be better translated as make or something more idiomatic.

Yes.

That version uses the vera að + infinitive construction:

• Ég er að gera þetta fyrir þig

This more clearly means:

• I am doing this for you
• I’m in the process of doing this for you

So the difference is roughly:

• Ég geri þetta fyrir þig = I do this for you / I am doing this for you
• Ég er að gera þetta fyrir þig = I am doing this for you right now / in progress

The simple present is very common, but the vera að form is useful when you want to stress an ongoing action.