Я учу русский не впустую: теперь мне легче говорить по-русски.

Because учить takes the accusative case when it means to study / learn something.

• учить русский = to learn/study Russian
• русский here stands for русский язык = the Russian language

Since язык is a masculine inanimate noun, its accusative singular looks like the nominative singular, so you get русский.

In this sentence, учу means I am learning / studying.

The verb учить can mean different things depending on context:

• учить русский = to learn/study Russian
• учить слова = to learn words
• учить детей = to teach children

So yes, учить can also mean to teach, but here the object русский clearly shows that the speaker is studying the language.

Yes, but the nuance is a little different.

• учу русский = the most natural everyday way to say I’m learning Russian
• изучаю русский = I’m studying Russian, often a bit more formal or academic

Both are correct, but учу русский sounds very normal in everyday speech.

Не впустую means not in vain, not for nothing, or not uselessly.

• впустую = in vain / to no purpose / uselessly
• не впустую = not in vain

So:

• Я учу русский не впустую = I’m not learning Russian for nothing

A close synonym is не зря.

Because впустую is an adverb, and with adverbs like this, не is often written separately when it simply negates the word:

• впустую = uselessly / in vain
• не впустую = not in vain

In this sentence, the meaning is a straightforward negation, so separate spelling is natural.

The colon shows that the second part explains or justifies the first part.

• Я учу русский не впустую = I’m not learning Russian for nothing
• теперь мне легче говорить по-русски = now it’s easier for me to speak Russian

So the second clause gives the reason or proof for the first one.

You could think of it as:

• ... not in vain: now...
• ... not in vain, because now...

Because Russian often uses a dative experiencer in sentences like it is easy/hard for someone.

• мне = to me / for me
• легче = easier

So мне легче literally means to me, [it is] easier, and naturally in English we say it’s easier for me.

This structure is very common:

• мне трудно = it’s hard for me
• ему интересно = it’s interesting to him / he finds it interesting
• нам легко = it’s easy for us

Легче is the comparative form of легко or лёгкий, depending on how you analyze it in context.

Here it means easier in an impersonal structure:

• легко = easily / easy
• легче = more easily / easier

In мне легче говорить, it works like it is easier for me to speak.

Russian often uses this kind of comparative without saying an explicit it.

Because after words like легче, Russian usually uses the infinitive to say what is easier.

• мне легче говорить = it’s easier for me to speak
• ему трудно понять = it’s hard for him to understand
• нам приятно видеть = it’s pleasant for us to see

If you said говорю, that would be a finite verb (I speak / I am speaking), which does not fit this structure.

Because по-русски is the adverb meaning in Russian / in the Russian language.

• говорить по-русски = to speak Russian
• писать по-русски = to write in Russian

By contrast, русский in учу русский is short for русский язык, the noun phrase the Russian language.

So the sentence uses two different patterns:

• учу русский = learn Russian
• говорить по-русски = speak in Russian / speak Russian

Yes, you often can.

Both are used, but they are not always identical in feel:

• говорить по-русски = the very common way to mean to speak Russian
• говорить на русском = also possible, often emphasizing the language used

In this sentence, говорить по-русски is especially idiomatic and natural.

A learner should definitely remember the pattern:

• по-английски
• по-русски
• по-французски

Because Russian often leaves out the dummy subject it that English requires.

English says:

• Now it is easier for me to speak Russian

Russian simply says:

• Теперь мне легче говорить по-русски

There is no need for a separate word corresponding to English it here. This is a normal impersonal construction.

Теперь means now.

It shows a change from before to the present:

• before, speaking Russian was harder
• now, it has become easier

So the full idea is something like:

• I’m not learning Russian for nothing: now it’s easier for me to speak Russian.

It highlights progress and improvement.

They are related, but they do different jobs in the sentence.

1. русский in учу русский
   This is basically an adjective standing in for русский язык = the Russian language.

2. по-русски in говорить по-русски
   This is an adverb, meaning in Russian or in a Russian way of speaking.

So:

• учу русский = I am learning Russian
• говорить по-русски = to speak Russian / to speak in Russian

English uses Russian in both places, but Russian uses different forms.