Ég segi alltaf satt.

Segja is the infinitive, meaning to say / to tell.
In the sentence, the verb has to match ég = I, so Icelandic uses the 1st person singular present tense form:

• ég segi = I say / I tell
• þú segir = you say / tell
• hann / hún / það segir = he / she / it says
• við segjum = we say
• þið segið = you say
• þeir / þær / þau segja = they say

So Ég segi alltaf satt is correct because segi matches ég.

Alltaf means always.

In a simple main clause like this, adverbs such as alltaf often come after the finite verb:

• Ég segi alltaf satt

That word order is very natural in Icelandic.

You can sometimes move alltaf for emphasis, but then Icelandic word order changes because the verb still tends to stay in the second position:

• Alltaf segi ég satt

That version is more marked or emphatic. For a neutral statement, Ég segi alltaf satt is the normal choice.

Satt is the neuter singular form of the adjective sannur, which means true.

In Icelandic, the expression segja satt is a very common way to say tell the truth or speak truthfully. Here, satt is used in a fixed, adverb-like expression after segja.

So learners should usually think of segja satt as a set phrase:

• segja satt = tell the truth

Because sattur is the masculine form of the adjective, and that is not what is needed here.

Compare:

• Ég er sannur would mean something like I am true in a masculine form, which is not the idea here.
• Ég segi satt means I tell the truth

So satt is not describing ég directly. It belongs to the expression segja satt.

Yes. That is also correct.

• Ég segi alltaf satt = I always tell the truth
• Ég segi alltaf sannleikann = I always tell the truth

The difference is mainly stylistic:

• segja satt is a very common, compact expression
• segja sannleikann uses the noun sannleikann = the truth and can sound a little fuller or more explicit

Both are natural Icelandic.

Normally, you should include ég.

Unlike languages such as Spanish or Italian, Icelandic usually does not regularly drop the subject pronoun in ordinary sentences. So the normal full sentence is:

• Ég segi alltaf satt

You may sometimes hear or see the subject omitted in very informal speech, diaries, poetry, or where the context makes it obvious, but that is not the standard form learners should rely on.

The sentence follows a very common Icelandic main-clause pattern:

• Ég = subject
• segi = finite verb
• alltaf = adverb
• satt = complement / fixed expression with the verb

So the order is:

Subject + Verb + Adverb + Complement

More importantly, Icelandic main clauses usually follow the verb-second principle. That means the finite verb tends to come in the second slot of the sentence.

Here, the first slot is Ég, so the second slot is segi.

Here it expresses a general habit or general truth about the speaker.

So Ég segi alltaf satt means something like:

• I always tell the truth
• I am someone who always tells the truth

It is not mainly describing one specific action happening at this exact moment. Icelandic present tense, like English simple present, is often used for habits and regular behavior.

A rough learner-friendly pronunciation is:

yeg SEY-yi AL-taf saht

A few helpful notes:

• Ég begins with a y sound
• g in ég is soft
• segi has two syllables: SEY-yi
• alltaf has the stress on the first syllable: AL-taf
• satt has a short a, roughly like sah t

As in Icelandic generally, the first syllable of each word is stressed.