Учитель пишет на доске мелом, когда маркер не работает.

Пишет is the imperfective present-tense form of писать.

That is used because the sentence describes a general/habitual action:

• Учитель пишет на доске мелом, когда маркер не работает.
• The idea is the teacher writes with chalk whenever the marker doesn’t work.

If you used the perfective verb написать, it would not fit here, because perfective verbs in Russian do not normally have a true present tense. Their present-looking forms usually refer to the future.

So here пишет means:

• writes
• is writing only in some contexts
• but in this sentence most naturally writes / uses chalk

Because на can take different cases depending on the meaning.

Here на доске means on the board as a location, so доска goes into the prepositional case:

• доска = board
• на доске = on the board

Compare:

• на доске = on the board (location)
• на доску = onto the board (direction)

So:

• Учитель пишет на доске = The teacher writes on the board.
• If you said something like вешает плакат на доску, that would mean putting something onto the board.

Because мелом is the instrumental case, which is often used to express the tool or means by which something is done.

• мел = chalk
• мелом = with chalk

So:

• писать мелом = to write with chalk
• писать ручкой = to write with a pen
• резать ножом = to cut with a knife

For an English speaker, this is a very common Russian pattern: instead of always using a separate word like with, Russian often uses the instrumental ending.

Usually, no—not if you mean using chalk as the writing instrument.

• писать мелом = to write with chalk
• с мелом usually means together with chalk or having chalk, not using chalk

So in this sentence, мелом is the normal and correct choice.

A native speaker expects the instrumental here because it marks the instrument directly.

Here когда means when in the sense of whenever / in situations where.

So the sentence means something like:

• The teacher writes on the board with chalk when the marker doesn’t work.
• Or more naturally in English: The teacher uses chalk on the board when the marker doesn’t work.

It is not really while here.
The focus is not on two actions happening at the same time, but on a condition-like repeated situation:

• marker doesn’t work → teacher uses chalk

In many cases, Russian когда can overlap with English when, whenever, and sometimes while, depending on context.

Because когда маркер не работает is a subordinate clause.

Russian normally separates subordinate clauses with commas:

• Учитель пишет на доске мелом, когда маркер не работает.

This is standard punctuation.
A good rule for learners is:

• If you see conjunctions like когда, потому что, если, чтобы, хотя, there is often a comma before the subordinate clause.

In Russian, работать is used not only for people but also for machines, devices, and tools that function properly.

So:

• Я работаю = I work
• Телефон не работает = The phone isn’t working
• Компьютер работает = The computer works
• Маркер не работает = The marker doesn’t work

This is completely normal Russian usage.

English sometimes uses work for objects too, so this is actually a helpful similarity.

Because Russian has no articles.

So учитель can mean:

• a teacher
• the teacher
• sometimes just teacher in a general sense

The exact meaning depends on context.

In this sentence, English could translate it as:

• The teacher writes on the board with chalk when the marker doesn’t work.
• Or A teacher writes on the board with chalk when the marker doesn’t work.

If the meaning was already given, the Russian itself does not tell you directly whether it is a or the.

Grammatically, учитель is a masculine noun and usually refers to a male teacher.

For a female teacher, everyday Russian often uses:

• учительница

However, in some formal or job-title contexts, masculine professional nouns can sometimes be used more generally. But for a learner, the simplest rule is:

• учитель = male teacher
• учительница = female teacher

You can also see the masculine form used in examples as a kind of default dictionary form.

Because the sentence describes a general repeated situation.

• Учитель пишет... = The teacher writes / uses chalk
• когда маркер не работает = when the marker doesn’t work

Russian often uses the present tense for:

• habits
• repeated situations
• general truths

So this is not necessarily about one specific moment right now. It is more like:

• Whenever the marker fails, the teacher writes with chalk.

If you wanted a specific past situation, you would change both clauses accordingly.

Russian word order is fairly flexible, although some versions sound more natural than others.

The original sentence is very natural:

• Учитель пишет на доске мелом, когда маркер не работает.

You could also say:

• Когда маркер не работает, учитель пишет на доске мелом.

This version puts more emphasis on the condition/situation first.

You might also hear slight rearrangements like:

• Учитель мелом пишет на доске, когда маркер не работает.

That is possible, but the original order sounds more neutral.

So yes, the word order can change, but the choice affects focus and style more than basic meaning.

Both are possible:

• пишет на доске мелом
• пишет мелом на доске

Russian word order is flexible, so both can be grammatical.

The difference is mainly focus:

• пишет на доске мелом first mentions where the writing happens, then with what
• пишет мелом на доске first highlights the instrument a bit more

The original sentence sounds natural and neutral.
A learner should not worry too much here: both orders are understandable, but the original is a good model to copy.

Here is a case breakdown:

• Учитель — nominative
  the subject of the sentence

• на доске — prepositional after на for location
  доска → доске

• мелом — instrumental
  means/instrument: with chalk

• маркер — nominative
  the subject of the subordinate clause

This sentence is a nice example of how Russian uses cases to show relationships that English often shows with prepositions and word order.

A natural way to think about it is:

• When the marker isn’t working, the teacher uses chalk to write on the board.

That helps you avoid translating too literally.

Word for word, it looks like:

• Учитель = teacher
• пишет = writes
• на доске = on the board
• мелом = with chalk
• когда = when
• маркер = marker
• не работает = does not work

But the natural idea is not just a list of words—it is a common classroom situation: the marker fails, so the teacher writes with chalk instead.