Jeg er altid opmærksom på små fejl i teksten.

In a normal Danish main clause, the finite verb usually comes in second position. Here, the finite verb is er, so it comes right after Jeg:

• Jeg er altid opmærksom på små fejl i teksten.

The adverb altid usually comes after the finite verb and before the rest of the sentence.

A very natural pattern is:

• subject + finite verb + adverb + rest

For example:

• Jeg er altid træt om morgenen.
• Hun kommer ofte for sent.

Putting altid in a different place is usually less natural or changes the emphasis.

Because opmærksom normally goes together with the preposition på when it means aware of / attentive to something.

So in Danish, you learn it as a fixed combination:

• at være opmærksom på noget = to be aware of / attentive to something

Examples:

• Vær opmærksom på trafikken.
• Jeg er opmærksom på problemet.

This is one of those verb/adjective + preposition combinations that you mostly have to memorize.

Here opmærksom describes jeg, and with jeg er ... it works as a predicate adjective.

In this sentence, the default common form is used:

• Jeg er opmærksom.

You often see -t on adjectives with et-words when the adjective directly describes a noun:

• et opmærksomt barn

But after at være with a person, you normally just use:

• Han er opmærksom.
• Hun er opmærksom.
• Jeg er opmærksom.

So opmærksomt would not fit here.

Because små is already the correct plural form of lille.

The adjective lille is irregular:

• singular common: lille
• singular neuter: lille
• plural: små

So:

• en lille fejl = a small mistake
• et lille problem = a small problem
• små fejl = small mistakes

That is why små fejl is correct.

Because fejl is one of the Danish nouns whose plural is often the same as the singular in the indefinite form.

So:

• en fejl = a mistake
• flere fejl = several mistakes

The definite plural is different:

• fejlene = the mistakes

So in your sentence:

• små fejl = small mistakes

That is indefinite plural, so fejl stays the same.

Because Danish often uses indefinite plural without an article, just like English does.

Compare:

• small mistakes
• små fejl

If you wanted the small mistakes, you would say:

• de små fejl

If you wanted to make it sound more like some small mistakes, Danish often still just says:

• små fejl

So the sentence is talking about small mistakes in general, not specific ones already identified.

Because when Danish talks about something being in a text, the normal preposition is i.

• i teksten = in the text

This matches the idea that the mistakes are contained within the text.

Examples:

• Der er mange fejl i teksten.
• Jeg fandt et problem i artiklen.

Using på would sound unnatural here. På is used in many other contexts, but not normally for errors inside a text.

Because teksten is the definite form: the text.

In Danish, definiteness is often shown by adding an ending to the noun:

• en tekst = a text
• teksten = the text

So:

• i teksten = in the text

This suggests a specific text, probably one already known from context.

If you said:

• i en tekst

that would mean in a text, more general or indefinite.

Yes, and it is very natural, but the nuance is a little different.

• Jeg er altid opmærksom på små fejl i teksten
  = I am always attentive to / aware of small mistakes in the text.

• Jeg lægger altid mærke til små fejl i teksten
  = I always notice small mistakes in the text.

So:

• er opmærksom på focuses more on your state of attentiveness
• lægger mærke til focuses more on the act of noticing

Both are good Danish, but they are not perfectly identical.

Yes. The verb er is the present tense of at være.

So the sentence describes a general habit or characteristic in the present:

• Jeg er altid ... = I am always ...

It does not necessarily mean only right now; it often means in general.

It is understandable, but it sounds much less natural in ordinary Danish.

The most natural placement is:

• Jeg er altid opmærksom på små fejl i teksten.

Putting altid at the very end can sound marked or unnatural unless you are emphasizing it in a special context.

So for normal speech and writing, keep altid after the finite verb:

• Jeg er altid ...

It is very common, especially in the pattern være opmærksom på.

Examples:

• Vær opmærksom på døren.
  Be aware of the door.

• Hun er opmærksom på detaljerne.
  She is attentive to the details.

• Vi skal være opmærksomme på problemet.
  We must be aware of the problem.

Notice that in the last example, opmærksom becomes opmærksomme because it refers to vi and is plural.

So the word is very useful, and the key thing to remember is the combination:

• opmærksom på