Mój brat używa golarki rano, a ja w tym czasie robię herbatę.

Because the verb używać normally takes the genitive case, not the accusative.

• golarka = nominative
• golarki = genitive singular
• golarkę = accusative singular

So:

• używać golarki = to use a shaver
• but mam golarkę = I have a shaver

This is one of the important things to memorize with Polish verbs: some verbs require a specific case.

Mój brat is in the nominative case, because it is the subject of the sentence.

• mój = my
• brat = brother

Together, mój brat means my brother as the person doing the action.

The form mój matches brat in:

• gender: masculine
• number: singular
• case: nominative

No, Polish often leaves out subject pronouns when they are clear from the verb form or context.

So instead of saying:

• Mój brat on używa...

Polish normally just says:

• Mój brat używa...

Also, once mój brat is already stated, adding on would usually be unnecessary.

Rano means in the morning or morning-time.

In Polish, it works as an adverb, so you do not need a preposition like w here.

• rano = in the morning
• wieczorem = in the evening
• nocą = at night

So używa golarki rano is a natural way to say he uses the shaver in the morning.

Here a is close to and, but it often has a slight sense of contrast or whereas.

So:

• Mój brat używa golarki rano, a ja...

feels like:

• My brother uses the shaver in the morning, and I, meanwhile...
• or ..., whereas I...

Polish has both i and a:

• i = simple and
• a = and/but/whereas, often linking two different subjects or contrasting actions

In this sentence, a is very natural because the speaker is contrasting what the brother does with what I do.

That is true: Polish often drops pronouns. But here ja is included for contrast.

• mój brat ...
• a ja ...

This gives the feeling of my brother does this, and I do that.

Without ja, the sentence would still be grammatical if the context were clear, but the contrast would be weaker. So ja is there mainly for emphasis.

After w in this meaning, Polish uses the locative case.

So:

• ten czas = this time
• w tym czasie = at that time / during that time / meanwhile

Here:

• tym is the locative form of ten
• czasie is the locative form of czas

This phrase is very common and means something like during that time or meanwhile.

Yes, literally it looks like I do tea, but idiomatically it means I make tea or I prepare tea.

In Polish, robić is used in many everyday expressions where English prefers make:

• robić herbatę = make tea
• robić kawę = make coffee
• robić obiad = make lunch / cook lunch

So this is a normal, natural Polish expression.

Also, herbatę is in the accusative case, because it is the direct object of robię.

Yes, both are present tense forms:

• używa = he uses / is using
• robię = I make / am making

In this sentence, because of rano and the general feel of the statement, it most naturally sounds like a habit or usual routine:

• My brother uses a shaver in the morning, and during that time I make tea.

But Polish present tense can also describe something happening now, depending on context.

The verbs używać and robić are imperfective, which fits this sentence well because it describes:

• repeated actions
• routine actions
• ongoing actions

Imperfective is the normal choice for habits such as in the morning.

If you used a perfective verb in the present, it would usually refer to the future in Polish, not a present habit.

For example:

• zrobię herbatę = I will make tea

So robię herbatę is correct for a present routine.

Yes. Polish word order is fairly flexible, and changing it usually changes emphasis, not the core meaning.

For example, you could also say:

• Rano mój brat używa golarki, a ja w tym czasie robię herbatę.

This puts more emphasis on rano.

The original sentence has a neutral, natural order:

• subject
• verb
• object
• time
• contrast
• second subject
• time phrase
• verb
• object

So the version you were given is very normal everyday Polish.

Yes. Golarka usually means shaver or electric razor.

Depending on context, it often suggests an electric device rather than a simple manual razor. If someone wanted to say a regular razor blade razor, they might use a different word such as maszynka do golenia.

So używa golarki most naturally suggests he uses a shaver/electric razor.