Learn Hardcore Greek: Ο καθηγητής, ο οποίος είναι πολύ συναισθηματικός, μας λέει συχνά προσωπικές ιστορίες. - The professor male, who is very emotional, often tells us personal stories.

Register:
- που is more common and colloquial.
- ο οποίος is more formal / written or “careful” style.
Clarity of reference:
ο οποίος changes form for gender, number, and case, so it can be clearer exactly which word it refers to in more complex sentences.

You could also say:

Ο καθηγητής, που είναι πολύ συναισθηματικός, μας λέει συχνά προσωπικές ιστορίες.

Same meaning, a bit more natural in everyday speech.

Why are there commas around ο οποίος είναι πολύ συναισθηματικός?

The commas mark a non‑restrictive (non‑defining) relative clause.

Ο καθηγητής, ο οποίος είναι πολύ συναισθηματικός, μας λέει…
= “The teacher, who is very emotional, often tells us personal stories.”

This clause adds extra information about the teacher, but does not tell you which teacher (you already know which teacher is meant).

If the clause were restrictive (defining which teacher), in everyday Greek you’d more likely see που and often no commas:

Ο καθηγητής που είναι πολύ συναισθηματικός μας λέει συχνά προσωπικές ιστορίες.
= “The teacher who is very emotional (as opposed to some other teacher) often tells us personal stories.”

Why is ο οποίος masculine nominative singular? How does the agreement work?

ο οποίος must agree in gender, number, and case with the noun it refers to, and also match its function inside the relative clause.

Antecedent: ο καθηγητής
- masculine, singular, nominative (subject of the main verb λέει)
Relative pronoun: ο οποίος
- masculine, singular, nominative, because inside the clause ο οποίος είναι πολύ συναισθηματικός it is the subject of είναι.

So:

Masculine (because καθηγητής is masculine)
Singular (because there is one teacher)
Nominative (because in the relative clause, “who” is the subject: “who is very emotional”)

Can I drop the article and say Ο καθηγητής, οποίος είναι πολύ συναισθηματικός, …?

No. In standard Modern Greek, ο οποίος is treated as a fixed form: the article is part of the word. You cannot normally drop it.

Correct forms (masc. sing. nom.):

ο οποίος, not οποίος on its own in this position.

The relative pronoun paradigm is:

Masculine: ο οποίος, του οποίου, τον οποίο, οι οποίοι, των οποίων, τους οποίους
Feminine: η οποία, της οποίας, την οποία, οι οποίες, των οποίων, τις οποίες
Neuter: το οποίο, του οποίου, το οποίο, τα οποία, των οποίων, τα οποία

What nuance does συναισθηματικός have? Is it like “emotional” or “sensitive”?

συναισθηματικός most directly corresponds to “emotional”:

Someone who feels emotions strongly, reacts with emotion, may express feelings easily.
Depending on context, it can be neutral, positive, or mildly negative.

Possible English translations:

“emotional” (most direct)
sometimes “sentimental”
occasionally “sensitive”, if the nuance is that the person is easily moved.

In this sentence, it’s neutral: the teacher is someone who feels and shows strong emotions, which fits with him often telling personal stories.

Why is μας placed before λέει instead of after, like “tells us”?

In Greek, weak object pronouns (clitics) like με, σου, τον, μας, σας, τους usually go before the verb in simple statements.

So:

μας λέει = “tells us”
Not λέει μας in standard word order.

Examples:

Μου δίνει το βιβλίο. = “He gives me the book.”
Σας γράφω συχνά. = “I write to you often.”

They move to other positions in questions, imperatives, etc., but in a neutral declarative sentence they stand right before the verb.

What is the grammatical role of μας in μας λέει συχνά προσωπικές ιστορίες?

Breaking down μας λέει συχνά προσωπικές ιστορίες:

(ο καθηγητής) – subject (he)
μας – weak object pronoun, indirect object: “to us”
λέει – verb “he tells”
συχνά – adverb “often”
προσωπικές ιστορίες – direct object: “personal stories”

So the structure is:

“[He] tells us personal stories often.”

Greek uses λέω κάτι σε κάποιον (“say/tell something to someone”), and μας is the “to us” part, but in form it’s just an accusative pronoun before the verb.

Where can συχνά go in the sentence? Are there other correct word orders?

Yes, adverbs like συχνά (“often”) are quite flexible. All of these are grammatical:

Ο καθηγητής, ο οποίος είναι πολύ συναισθηματικός, μας λέει συχνά προσωπικές ιστορίες.
Ο καθηγητής, ο οποίος είναι πολύ συναισθηματικός, συχνά μας λέει προσωπικές ιστορίες.
Ο καθηγητής, ο οποίος είναι πολύ συναισθηματικός, μας συχνά λέει προσωπικές ιστορίες. (less common, somewhat marked)
Ο καθηγητής, ο οποίος είναι πολύ συναισθηματικός, μας λέει προσωπικές ιστορίες συχνά.

Typical, natural choices are 1 and 2.
Position closer to the verb (μας λέει συχνά) sounds very neutral. Moving συχνά around can add slight emphasis (e.g. συχνά μας λέει stresses the frequency a bit more).

How would I say “The teacher, who used to be very emotional, often told us personal stories” in Greek? Do I still use λέει?

You need the past tense (imperfect) of λέω for “used to tell / often told” and past for είναι:

Ο καθηγητής, ο οποίος ήταν πολύ συναισθηματικός, μας έλεγε συχνά προσωπικές ιστορίες.

Changes:

είναι → ήταν (“is → was”)
λέει → έλεγε
- imperfect: repeated / habitual action in the past (“often told / used to tell”)

What exactly does προσωπικές ιστορίες mean? Is it “personal” as in private, or “about his personal life”?

προσωπικές ιστορίες = “personal stories”.

In Greek, προσωπικός / προσωπική / προσωπικό often implies:

Stories about one’s own life / experiences, or
Stories that are intimate / private in some way.

So here it most naturally suggests:

“Stories from his own life”
or
“Private, personal stories” (not just neutral, impersonal anecdotes).

Why do προσωπικές and ιστορίες both end in -ες? How does agreement work here?

In προσωπικές ιστορίες, the adjective προσωπικές must agree with ιστορίες in:

Gender:
ιστορίες is feminine plural → προσωπικές is feminine plural.
Number:
plural → plural
Case:
accusative plural → accusative plural

Forms:

Singular:
- μια προσωπική ιστορία (“a personal story”)
Plural:
- προσωπικές ιστορίες (“personal stories”)

So the matching endings -ές show feminine accusative plural agreement.

If the teacher were female, how would the sentence change?

You’d change the noun and all the words that must agree with it in gender (and possibly in form):

Η καθηγήτρια, η οποία είναι πολύ συναισθηματική, μας λέει συχνά προσωπικές ιστορίες.

Changes:

Ο καθηγητής → Η καθηγήτρια (feminine)
ο οποίος → η οποία (feminine relative pronoun)
συναισθηματικός → συναισθηματική (feminine adjective)

The rest stays the same, because μας, λέει, συχνά, προσωπικές ιστορίες don’t depend on the teacher’s gender.

Could I use another verb instead of μας λέει for “tells us personal stories”? For example, μας διηγείται?

Yes. λέω is very general and common, but for “telling stories” you can also use:

μας διηγείται προσωπικές ιστορίες
μας αφηγείται προσωπικές ιστορίες

Nuances:

λέω – very general “say / tell”.
διηγούμαι / αφηγούμαι – “narrate, recount”; more specific for telling a story in some detail, often a bit more formal/literary.

So:

Ο καθηγητής, ο οποίος είναι πολύ συναισθηματικός, μας διηγείται συχνά προσωπικές ιστορίες.
= “The teacher, who is very emotional, often recounts personal stories to us.”