Learn Hardcore German: Manchmal frage ich mich, ob dieses Training überhaupt Sinn hat. - Sometimes I ask myself whether this training makes any sense at all.

fragen + Akkusativ: to ask someone a question
- Ich frage dich. = I ask you.
sich fragen (reflexive): to wonder, to ask oneself
- Ich frage mich. = I wonder / I ask myself.

In your sentence, you are not asking another person, you are wondering about something yourself. So you must use the reflexive form ich frage mich.

Using ich frage alone would sound incomplete, because German expects an object: Ich frage (jemanden / nach etwas).

Why is there a mich in frage ich mich? Who is doing what to whom?

mich is a reflexive pronoun. It refers back to the subject ich:

Subject: ich (I)
Reflexive object: mich (myself)

Literally, ich frage mich is “I ask myself.” German uses this reflexive structure to express to wonder.

So the internal structure is:

ich (subject)
frage (verb)
mich (reflexive object = the same person as the subject)

Why does the sentence start with Manchmal frage ich mich and not Ich frage mich manchmal?

Both are correct:

Manchmal frage ich mich, ob …
Ich frage mich manchmal, ob …

The difference is only in emphasis and style:

Starting with Manchmal (“sometimes”) puts more emphasis on the frequency: Sometimes, I wonder if…
Saying Ich frage mich manchmal sounds a bit more neutral and closer to English word order.

German word order rule: if something other than the subject starts the sentence (here: Manchmal), the finite verb (frage) must come second, and the subject (ich) moves after the verb:

Manchmal – frage – ich – mich …

Why is there a comma before ob?

In German, ob introduces a subordinate clause (a dependent clause). Subordinate clauses are always separated by a comma from the main clause.

Main clause: Manchmal frage ich mich
Subordinate clause: ob dieses Training überhaupt Sinn hat

So the comma marks the boundary between these two clauses:

Manchmal frage ich mich, ob dieses Training überhaupt Sinn hat.

What does ob mean here, and how is it different from wenn or falls?

In this sentence, ob means if or whether in an indirect yes/no question:

Ich frage mich, ob dieses Training überhaupt Sinn hat.
= I wonder if / whether this training makes any sense.

Use ob when:

The answer is yes/no, and
The question is embedded in a larger sentence (indirect question).

Examples:

Ich weiß nicht, ob er kommt. = I don’t know if he’s coming.
Kannst du mir sagen, ob das richtig ist? = Can you tell me if that is correct?

wenn / falls usually mean if in a conditional sense:

Wenn / Falls er kommt, freue ich mich. = If he comes, I’ll be happy.

So here ob is correct; wenn or falls would change the meaning to a condition, which is not intended.

Why does the verb go to the end in ob dieses Training überhaupt Sinn hat?

In German subordinate clauses, the finite verb goes to the end of the clause.

ob dieses Training überhaupt Sinn hat

Structure:

ob (subordinating conjunction)
dieses Training (subject)
überhaupt (adverb)
Sinn (object)
hat (finite verb at the end)

This is a standard word-order rule:

Main clause: Dieses Training hat überhaupt Sinn.
Subordinate clause with ob: … ob dieses Training überhaupt Sinn hat.

What does überhaupt add to the sentence? Could I leave it out?

überhaupt here adds emphasis and a slightly skeptical tone. Roughly:

ob dieses Training Sinn hat = if this training makes sense
ob dieses Training überhaupt Sinn hat = if this training makes any sense at all

It often conveys:

doubt or skepticism: maybe it doesn’t make sense
totality: at all, in any way, in the first place

You can leave it out grammatically:

Manchmal frage ich mich, ob dieses Training Sinn hat.

But then the sentence sounds a bit more neutral and slightly less doubtful. überhaupt makes the doubt stronger.

How does Sinn haben work? Why not say something like ist Sinn or hat einen Sinn?

The core idea is:

Sinn haben = to make sense, to be meaningful

Common patterns:

Das hat Sinn. = That makes sense / that is meaningful.
Dieses Training hat (überhaupt) Sinn. = This training (even) makes sense.

You can also say:

Das hat einen Sinn. = That has a (specific) purpose/point.

Using ist Sinn would be wrong in this context; Sinn here is not an adjective, it’s a noun meaning sense / meaning / purpose. Something has sense; it doesn’t is sense.

So the structure is:

Subject: dieses Training
Verb: hat
Object: (überhaupt) Sinn

Literally: this training has (any) sense → this training makes (any) sense.

Is Sinn machen (to make sense) wrong? I’ve heard people say Das macht Sinn.

In everyday German, Sinn machen is very common:

Das macht Sinn. = That makes sense.

Traditional prescriptive grammarians used to prefer Sinn ergeben or Sinn haben:

Das ergibt Sinn.
Das hat Sinn.

However, in modern usage, Sinn machen is widespread and generally accepted, especially in spoken language and informal writing.

In your sentence, Sinn haben is perfectly fine and maybe slightly more neutral. You could rephrase as:

… ob dieses Training überhaupt Sinn macht. (more colloquial)
… ob dieses Training überhaupt Sinn ergibt. (a bit more formal)

Why is it dieses Training and not diese Training?

Because Training in German is:

grammatical gender: neuter
article: das Training

The demonstrative dies- (“this”) is declined according to gender, case, and number.

In dieses Training, we have:

Case: Nominative (subject of the subordinate clause)
Number: singular
Gender: neuter

So the correct form is:

dieses (nominative singular neuter) + Training

A quick comparison:

dieser Mann (masc., nom. sg.)
diese Frau (fem., nom. sg.)
dieses Kind / dieses Training (neuter, nom. sg.)

In ob dieses Training überhaupt Sinn hat, which word is the subject and which is the object?

In that clause:

Subject: dieses Training
Verb: hat
Object: Sinn

So the basic structure is:

[Dieses Training] (subject) hat [Sinn] (object).

This can feel reversed compared to English, where we say:

This training makes sense.

In English, sense is the subject (“sense is being made by this training”), but in German the thing being evaluated (dieses Training) is the subject, and Sinn is what it “has.”

Is the tone/formality of this sentence neutral, formal, or informal?

The sentence is neutral and suitable in many contexts:

Everyday conversation: absolutely fine
Writing (emails, essays): also fine
Very formal academic or bureaucratic texts: still okay, though they might prefer alternatives like … ob dieses Training sinnvoll ist.

If you want a clearly formal/neutral variant, you could also say:

Manchmal frage ich mich, ob dieses Training überhaupt sinnvoll ist.
(using the adjective sinnvoll = meaningful, useful)