Manchmal irre ich mich, wenn ich zu schnell lese.

Because German main clauses usually follow the verb-second rule.

That means:

• the first position can be taken by one element, here Manchmal
• the conjugated verb must come next, here irre
• then comes the subject, here ich

So the structure is:

Manchmal | irre | ich | mich

If you started with Ich, then you would say:

Ich irre mich manchmal.

Both are correct, but the emphasis is slightly different:

• Manchmal irre ich mich = emphasis on sometimes
• Ich irre mich manchmal = more neutral

Because the verb is sich irren, which is a reflexive verb.

So German says:

• ich irre mich
• literally something like I mistake myself

But in natural English, it means:

• I am mistaken
• I make mistakes
• I’m wrong

The reflexive pronoun changes with the subject:

• ich irre mich
• du irrst dich
• er/sie irrt sich
• wir irren uns

So mich is there because sich irren requires a reflexive pronoun.

Here it appears in the reflexive form sich irren, which means:

• to be mistaken
• to be wrong
• to make a mistake

Be careful: irren by itself can also have other meanings in some contexts, such as to wander or to go astray, but that is not what is happening here.

In this sentence, the important expression is:

sich irren = to be mistaken

So Ich irre mich means I am mistaken or I make mistakes.

Because it is a subordinate clause introduced by wenn.

In German, subordinate clauses often push the conjugated verb to the end.

So:

• wenn = introduces the clause
• ich = subject
• zu schnell = adverbial phrase
• lese = verb, placed at the end

That is why you get:

wenn ich zu schnell lese

not

wenn ich lese zu schnell

Because wenn is a subordinating conjunction.

Subordinating conjunctions in German, such as:

• wenn
• weil
• dass
• obwohl

send the finite verb to the end of the clause.

Compare:

Main clause:

• Ich lese zu schnell.

Subordinate clause:

• wenn ich zu schnell lese

This is one of the most important German word-order patterns to learn.

Here wenn is being used in the sense of when/whenever.

It often refers to:

• repeated situations
• general conditions
• things that happen whenever something else happens

So this sentence means something like:

• Sometimes I make mistakes when I read too quickly
• or Sometimes I make mistakes if I read too quickly

In many cases, English separates when and if more clearly than German does. German wenn can cover both ideas in the right context.

A useful comparison:

• wenn = when/whenever/if for repeated or conditional situations
• als = when for a one-time event in the past

Example:

• Wenn ich müde bin, lese ich langsamer.
• Als ich gestern müde war, las ich langsamer.

Zu schnell means too fast or too quickly.

The word zu here means too in the sense of more than is good/helpful/appropriate.

So:

• schnell = fast
• zu schnell = too fast

That is different from:

• sehr schnell = very fast

So:

• Ich lese sehr schnell. = I read very fast.
• Ich lese zu schnell. = I read too fast.

In your sentence, the idea is that reading excessively fast causes mistakes.

Because in German, subordinate clauses are normally separated from the main clause by a comma.

So the comma before wenn ich zu schnell lese is required.

This is very standard in German:

• Ich bleibe zu Hause, weil ich müde bin.
• Sie ruft an, wenn sie Zeit hat.
• Manchmal irre ich mich, wenn ich zu schnell lese.

German uses commas more systematically with subordinate clauses than English does.

Yes, absolutely.

That version is also correct:

• Ich irre mich manchmal, wenn ich zu schnell lese.

The difference is mainly emphasis and rhythm.

Compare:

• Manchmal irre ich mich, wenn ich zu schnell lese.
  Emphasizes sometimes
• Ich irre mich manchmal, wenn ich zu schnell lese.
  More neutral; starts with the subject

German often allows this kind of flexibility because of the verb-second rule.

No. Manchmal is not a noun here.

It is capitalized only because it is the first word in the sentence.

If it appeared in the middle of a sentence, it would normally be lowercase:

• Ich irre mich manchmal.

So the capitalization is just normal sentence-initial capitalization.

Because there are actually two clauses, and each clause needs its own subject.

1. Main clause:

   • Manchmal irre ich mich

2. Subordinate clause:

   • wenn ich zu schnell lese

In English, you also repeat the subject:

• Sometimes I’m mistaken when I read too quickly.

So the second ich belongs to the wenn clause and is not optional.

It is in the present tense.

• irre = present tense of sich irren
• lese = present tense of lesen

German often uses the present tense for:

• habits
• general truths
• repeated actions
• things happening now

This sentence describes a general habit or repeated situation, so the present tense is the natural choice.

A common past version would be:

Manchmal habe ich mich geirrt, wenn ich zu schnell gelesen habe.

This uses the Perfekt tense:

• habe mich geirrt = have been mistaken / made mistakes
• gelesen habe = have read

Notice that in the subordinate clause, the verb elements still go to the end:

• wenn ich zu schnell gelesen habe

That is another example of German subordinate-clause word order.