Я тяжело вздохнул, потому что так и не смог толком объяснить свою мысль.

Because Russian past tense agrees with the gender of the subject in the singular.

• вздохнул = he/I (male) sighed
• вздохнула = she/I (female) sighed
• вздохнуло = it sighed
• вздохнули = they sighed

Since the speaker says Я, this form suggests the speaker is male. A female speaker would say Я тяжело вздохнула.

вздохнул is perfective, so it refers to one complete action: he/I gave a sigh.

вздыхал is imperfective, and would usually suggest:

• repeated sighing,
• a process,
• or background description.

So:

• Я тяжело вздохнул = I sighed heavily / I let out a heavy sigh
• Я тяжело вздыхал = I was sighing heavily / I kept sighing heavily

In this sentence, the speaker means one specific sigh caused by frustration, so the perfective verb fits better.

Here тяжело means heavily, not with difficulty.

So тяжело вздохнул means:

• sighed heavily
• let out a heavy sigh

Russian тяжело can mean different things depending on context:

• Мне тяжело идти = It is hard for me to walk
• Он тяжело дышит = He is breathing heavily
• Я тяжело вздохнул = I sighed heavily

So in this sentence it describes the manner of the sigh, not the difficulty of doing it.

Yes. Потому что is one of the most common and neutral ways to say because in Russian.

So:

• Я тяжело вздохнул, потому что... = I sighed heavily because...

It introduces the reason for the action.

Other Russian words for because exist, such as так как, but потому что is the most everyday and straightforward choice here.

This is a very common Russian pattern:

• так и не + past/perfective verb

It means something like:

• never did manage to
• still did not
• failed to, in the end
• didn’t succeed after all

So:

• так и не смог объяснить = never managed to explain
• так и не пришёл = never came after all
• так и не понял = still didn’t understand

The phrase often adds a sense of disappointment, incompletion, or failed expectation.

In this sentence, так и не смог is stronger than just не смог. It suggests that the speaker tried or wanted to explain the thought, but in the end failed.

Because смочь means to be able / to manage, and it is commonly followed by an infinitive.

So:

• смог объяснить = managed to explain / was able to explain
• не смог объяснить = was not able to explain / failed to explain

This is similar to English structures like:

• could explain
• managed to explain
• failed to explain

Russian often uses this kind of two-verb structure with verbs of ability:

• мог понять = could understand
• смог сделать = managed to do
• не смог сказать = couldn’t say

Толком means something like:

• properly
• clearly
• coherently
• in a satisfactory way

It is somewhat colloquial and very common in speech.

So:

• толком объяснить = explain properly
• не понял толком = didn’t really understand properly
• не знаю толком = I don’t really know clearly

In this sentence, не смог толком объяснить свою мысль means not just I couldn’t explain my thought, but more specifically I couldn’t explain it clearly or properly.

Russian often uses the reflexive possessive свой when the possessor is the same as the subject of the clause.

Here the subject is Я, so:

• свою мысль = my own thought/idea

This is more natural in Russian than мою мысль in this sentence.

Compare:

• Я объяснил свою мысль = I explained my thought
• Он объяснил свою мысль = He explained his thought
• Я объяснил мою мысль sounds less natural here unless there is some special contrast or emphasis.

So свой is the normal choice when the thing belongs to the subject.

Because it agrees with мысль, and мысль is:

• feminine
• singular
• the direct object here

The verb объяснить takes a direct object in the accusative case, so своя мысль becomes свою мысль.

Forms of свой in the singular accusative include:

• masculine animate: своего
• masculine inanimate: свой
• feminine: свою
• neuter: своё

Since мысль is feminine, the form is свою.

Because for many feminine nouns of this type, the accusative singular looks the same as the nominative singular.

So:

• nominative: мысль
• accusative: мысль

You can still tell it is accusative from its role in the sentence and from the agreeing word свою.

This kind of pattern is normal for many feminine nouns ending in -ь.

Both are possible depending on context.

мысль can mean:

• thought
• idea
• point
• what one wants to say

In this sentence, explain my thought is understandable, but in natural English you might often say:

• explain my idea
• express what I meant
• get my point across

So the Russian is normal, even if the most natural English translation may vary.

Объяснить means to explain, so it focuses on making the thought understandable.

Выразить means to express, which focuses more on putting the thought into words.

Both are possible in similar situations, but they are not identical:

• не смог объяснить свою мысль = couldn’t explain my thought properly
• не смог выразить свою мысль = couldn’t express my thought properly

In this sentence, объяснить suggests the speaker could not make the idea clear enough to the listener.

No, Russian word order is flexible, but the given order sounds natural.

The sentence is:

• Я тяжело вздохнул, потому что так и не смог толком объяснить свою мысль.

This order presents the sigh first, then the reason for it. Inside the second clause, так и не смог is kept together because it is a set-like pattern, and толком naturally goes before объяснить.

You could rearrange parts for emphasis, but not every version sounds equally natural. The original word order is smooth and idiomatic.

Yes, often it could be omitted if the context already makes the subject clear:

• Тяжело вздохнул, потому что так и не смог толком объяснить свою мысль.

Russian often drops subject pronouns when they are understood from the verb form and context.

However, Я may be kept:

• for clarity,
• at the start of a story,
• or for mild emphasis.

So both versions are possible, but the full version with Я is completely normal.