Learn Hardcore Russian: Я надеваю наушники, чтобы громкий голос по радио не мешал моим мыслям. - I put on headphones so that the loud voice on the radio doesn’t disturb my thoughts.

надевать / надеть – to put something on (yourself)
- Я надеваю наушники. – I am putting on headphones.
- Я надену наушники. – I will put on headphones (one-time future).
носить – to wear (or to carry) as a state, not the action of putting on
- Я ношу наушники. – I (usually) wear headphones.
одевать / одеть – to dress someone else
- Мама одевает ребёнка. – Mom is dressing the child.

So here, the action is “I put on headphones (now/in general)”, so надеваю is correct.
Ношу would mean “I wear headphones (in general)”, and одеваю would mean “I am dressing (someone)”, which doesn’t fit.

Why is наушники plural? Is there a singular form in Russian?

Russian usually uses наушники in the plural to mean “headphones / earphones” as an object with two earpieces.

наушники – headphones (normal, general word, always looks plural)
There is a singular наушник, but:
- it often means one earpiece (e.g. a single earbud), or
- it appears in technical contexts.

In everyday speech, when you say я надеваю наушники, people understand “I put on (my) headphones / earphones” and it sounds completely natural. Using the plural is the default.

What case is наушники here, and how can I see it?

In Я надеваю наушники, the word наушники is in the accusative plural, because it is the direct object of the verb надеваю (“I put on what? – headphones”).

For inanimate masculine and neuter nouns, the accusative plural looks the same as the nominative plural. So наушники could be either nominative plural or accusative plural in form.

You know it’s accusative here because of the role in the sentence:

Я надеваю (кого? что?) наушники. – object → accusative.

What does по радио literally mean, and why do we use по, not на?

по радио literally means something like “via radio / over the radio”.

The preposition по is commonly used for:

communication channels: по телефону (on the phone), по телевизору (on TV), по радио (on the radio)
movement along something: по дороге (along the road)

So:

громкий голос по радио ≈ “the loud voice (coming) over the radio”.

Using на радио usually refers to the radio station as a workplace or institution, e.g.:

Он работает на радио. – He works on the radio (at a radio station).

Here we’re talking about the signal / sound coming through the radio, so по радио is correct.

Why is there a comma before чтобы?

Чтобы introduces a subordinate clause of purpose (“so that / in order that”).

In Russian, subordinate clauses are normally separated from the main clause with a comma.
The structure here is:

Main clause: Я надеваю наушники
Purpose clause: чтобы громкий голос по радио не мешал моим мыслям.

So we write:

Я надеваю наушники, чтобы громкий голос по радио не мешал моим мыслям.

Why is it чтобы ... не мешал, with мешал looking like past tense? Why not a present form?

In Russian, after чтобы (when it means “so that / in order that”), we normally use the subjunctive: a past-tense form + the particle бы.

In this construction, бы is absorbed into чтобы, so you don’t add it separately:

underlying structure: … чтобы (бы) голос не мешал …

The form мешал is grammatically past tense, masculine, singular, but in this context it expresses something like:

“so that the loud voice on the radio would not disturb my thoughts.”

So:

English: “I put on headphones so that the loud voice on the radio doesn’t / won’t / wouldn’t bother my thoughts.”
Russian: Я надеваю наушники, чтобы громкий голос по радио не мешал моим мыслям.

It’s not “past” in meaning here; it’s the standard subjunctive form after чтобы.

How does the verb мешать work? Why is it моим мыслям, not мои мысли?

The verb мешать (“to disturb / to bother / to interfere”) can take a dative object for the thing/person that is being bothered:

мешать кому? чему? – to bother whom? what? (in dative)

Examples:

Шум мешает мне. – The noise bothers me.
Музыка мешает работе. – The music interferes with (my) work.

In the sentence:

громкий голос по радио – subject (nominative)
мешал – verb
моим мыслям – indirect object (dative plural), answering чему? (what is being bothered?)

So we use the dative form моим мыслям, not the nominative мои мысли.

What exactly is the form моим мыслям? How is it built?

Моим мыслям is dative plural for “my thoughts”:

мысль – thought (feminine, singular)
мысли – thoughts (nominative plural)
мыслям – to thoughts (dative plural)

The possessive pronoun мой must agree in gender, number, and case:

мой (m. sg. nom.), моя (f. sg. nom.), моё (n. sg. nom.), мои (pl. nom.)
Dative plural of мой is моим.

So:

моим мыслям = to my thoughts (dative plural), matching the verb мешать (мешать кому? чему?).

Why is it громкий голос, not громкого голоса?

Громкий голос is in the nominative case because it is the subject of the verb мешал:

(Кто? Что?) громкий голос по радио не мешал (кому? чему?) моим мыслям.

Nominative is the default case for the subject of a sentence.

You would see громкого голоса (genitive) in structures like:

у меня нет громкого голоса – I don’t have a loud voice.

Here we are not saying “there is no loud voice”; we are saying “the loud voice (on the radio) does not disturb my thoughts”, so громкий голос must be nominative.

Can the word order in the second part be changed, e.g. чтобы громкий голос по радио моим мыслям не мешал?

Yes. Russian word order is relatively flexible, especially in longer sentences. All of these are grammatically possible:

… чтобы громкий голос по радио не мешал моим мыслям.
… чтобы громкий голос по радио моим мыслям не мешал.
… чтобы моим мыслям не мешал громкий голос по радио.

The basic roles are marked by case endings, not by position:

громкий голос по радио – nominative → subject
моим мыслям – dative → thing being bothered

The original order is the most neutral: subject → verb → (indirect) object. Other orders can add slight emphasis (e.g. moving моим мыслям earlier to stress my thoughts), but they all convey the same core meaning.