Learn Hardcore French: Le professeur souligne les fautes d’orthographe avec un stylo rouge. - The teacher underlines the spelling mistakes with a red pen.

We are talking about a specific teacher that the speaker and listener both know (for example, the teacher of this class).
Or we are talking about the teacher in a general, habitual way:
Le professeur souligne les fautes… = The teacher (in general) underlines the mistakes… (this is what he/she typically does).

Un professeur souligne les fautes… would sound more like:

A teacher underlines the mistakes… (introducing some random or new teacher, not one already known from context).

So le matches English “the” here: a particular or contextually known teacher.

Why is the verb souligne and what tense is it?

The infinitive (dictionary form) is souligner = to underline, to highlight.

Here we have:

Subject: Le professeur (3rd person singular)
Verb: souligne

This is the present tense, 3rd person singular of a regular -er verb:

je souligne
tu soulignes
il / elle / on souligne
nous soulignons
vous soulignez
ils / elles soulignent

So Le professeur souligne… = The teacher underlines… (a present, repeated, or habitual action).

What exactly does souligne mean here? Underline, highlight, or “point out”?

Souligner literally means “to underline” (draw a line under words).

In practice, it can imply a few related ideas:

Physical action
- With a pen or marker, the teacher draws a line under the spelling mistakes.
- This is the most literal reading in this sentence, especially with un stylo rouge.
Metaphorical / figurative
- souligner can also mean “to emphasize, to stress” a point.
  Example: Je veux souligner l’importance de… = I want to stress the importance of…

Here, because we mention un stylo rouge, it’s almost certainly the physical underlining of spelling mistakes, not just “pointing them out” abstractly (for that, you might also see corriger = to correct, or signaler = to point out).

Why is it les fautes and not des fautes?

Both are possible, but the nuance is different:

les fautes d’orthographe = the spelling mistakes
→ We are thinking of all the specific mistakes in this text/homework.
des fautes d’orthographe = (some) spelling mistakes
→ We are introducing the existence of mistakes, not referring to a whole specific set.

In this sentence:

Le professeur souligne les fautes d’orthographe…

we imagine a particular piece of work with its errors. The teacher underlines those errors, so French uses the definite article les.

Compare:

Il y a des fautes d’orthographe. = There are (some) spelling mistakes.
Le professeur souligne les fautes d’orthographe. = The teacher underlines the spelling mistakes (those in the text).

What does d’orthographe mean, and why is it not de orthographe or de l’orthographe?

Orthographe means spelling.

Why “d’” and not “de”?
- de + orthographe → the vowel o at the start of orthographe
- In French, de becomes d’ before a vowel sound: d’orthographe.
  This is called elision.
Why “de” (or “d’”) at all?
faute d’orthographe is a fixed pattern faute de + noun, meaning “a mistake in X”:
- faute d’orthographe = a spelling mistake
- faute de grammaire = a grammar mistake
- faute de français = a mistake in French

So des fautes d’orthographe literally = mistakes of spelling, i.e. spelling mistakes.

We don’t normally say faute de l’orthographe here; faute d’orthographe is the standard expression.

Is faute d’orthographe singular or plural? What is agreeing with what?

Break the phrase down:

faute = mistake (feminine, singular noun)
fautes = mistakes (feminine, plural)
d’orthographe = of spelling (no plural s and no agreement here)

So in the full sentence:

les fautes → plural (-s)
d’orthographe → stays the same, regardless of singular/plural

Examples:

une faute d’orthographe = one spelling mistake
des fautes d’orthographe = some spelling mistakes
les fautes d’orthographe = the spelling mistakes

Only faute/fautes changes; orthographe stays the same.

Why is the adjective rouge placed after stylo and not before it?

In French, most adjectives go after the noun:

un stylo rouge = a red pen
une voiture rouge = a red car

Some common adjectives go before the noun (often remembered as the BANGS group: Beauty, Age, Number, Goodness, Size):

un beau stylo
une petite voiture
un bon professeur

But couleur adjectives like rouge, bleu, vert, noir, blanc, etc. almost always go after the noun:

un stylo rouge
une robe bleue
une voiture noire

So un stylo rouge follows the normal rule: noun + color adjective.

Does rouge change for gender and number here?

In the sentence:

avec un stylo rouge

we have:

stylo: masculine singular
un: masculine singular article
rouge: masculine singular form (no extra ending)

Rouge has two written forms:

Singular: rouge (for both masculine and feminine)
Plural: rouges

Examples:

un stylo rouge (m. sg.)
une robe rouge (f. sg.)
des stylos rouges (m. pl.)
des robes rouges (f. pl.)

So it does agree in number (add -s in the plural), but not in gender (same form for masculine and feminine singular).

Why is it avec un stylo rouge and not something like par un stylo rouge or en stylo rouge?

To express the means or instrument used to do something, French usually uses avec or à:

avec un stylo rouge = with a red pen
au stylo rouge = with a red pen (literally “at the red pen” − see next question)

par is usually “by” in the sense of author/agent or route:

un livre écrit par Victor Hugo = a book written by Victor Hugo
passer par la porte = go through the door

en is used differently (material, state, transport, etc.):

une statue en bois = a statue made of wood
en voiture = by car

So to say using a red pen, avec un stylo rouge is the normal and natural choice.

Could we also say au stylo rouge instead of avec un stylo rouge? Is there a difference?

Yes, you could say:

Le professeur souligne les fautes d’orthographe au stylo rouge.

Both are correct, but there’s a nuance:

avec un stylo rouge
Focus on the instrument: with a red pen (just stating the tool used).
au stylo rouge ( = à + le stylo rouge)
This is very common in instructions (teachers, exam papers):
Écrivez au stylo noir. = Write in black pen.
It can sound a bit more like a fixed way or standard method of doing it.

In everyday speech, avec un stylo rouge is perfectly natural; au stylo rouge is also standard and perhaps a bit more “school-style” or instructional in tone.

Why is professeur masculine? What if the teacher is a woman?

Historically, professeur has been treated as a masculine noun in French, even when talking about a woman:

Le professeur est sympa. Elle est très compétente.

However, modern usage increasingly accepts a feminine form:

une professeure (with an e at the end)
or informally: une prof (short for professeur)

Possible patterns today:

About a man:
- le professeur
- un professeur
- un prof
About a woman:
- traditional/neutral: le professeur (grammar masculine, person female)
- increasingly common: une professeure
- informal: une prof

In your sentence, Le professeur souligne… is grammatically masculine, but the actual person could be male or female; the verb form souligne doesn’t change.

How is this sentence pronounced? Are there any liaisons?

Approximate pronunciation (International Phonetic Alphabet):

Le professeur → /lə pʁɔ.fɛ.sœʁ/
souligne → /su.liɲ/
les fautes → /le fot/ (often with liaison /lez‿fot/ in careful speech)
d’orthographe → /d‿ɔʁ.tɔ.ɡʁaf/ (the d’ links into the vowel)
avec → /a.vɛk/
un stylo → /œ̃ sti.lo/
rouge → /ʁuʒ/

Two main liaison points to notice:

les fautes
- In more careful or formal speech, you may hear: les‿fautes /lez‿fot/
- In more relaxed speech, the liaison may be less marked.
d’orthographe
- There is an obligatory link between d’ and orthographe: /d‿ɔʁtɔɡʁaf/
  You won’t hear a separate /ə/ vowel for de: it’s contracted to d’.

Silent letters:

Final -s in fautes is silent.
Final -e in orthographe, avec is silent.
Final -e in rouge is silent (you only hear /ʁuʒ/).

Could we use another verb like corrige or surligne instead of souligne?

Yes, but the meaning changes slightly:

souligne (souligner)
- literally: to underline
- here: the teacher draws a line under the mistakes.
corrige (corriger)
- to correct
- The teacher doesn’t just mark them, but writes the correct form or fixes them.
surligne (surligner)
- to highlight (with a highlighter, usually above or around the word)
- More like using a fluorescent marker.

So:

Le professeur souligne les fautes…
The teacher underlines the mistakes.
Le professeur corrige les fautes…
The teacher corrects the mistakes.
Le professeur surligne les fautes…
The teacher highlights the mistakes (often with a marker).

Your original sentence focuses specifically on the act of underlining.