Learn Hardcore Greek: Όσα βιβλία διαβάζεις σε βοηθάνε πολύ.

Inside the subordinate clause: βιβλία = direct object of διαβάζεις.
In the main clause, the whole chunk Όσα βιβλία διαβάζεις acts as the subject.

Is Όσα βιβλία διαβάζεις really the subject of βοηθάνε? Why is the verb plural?

Yes. The subject of βοηθάνε is the entire phrase Όσα βιβλία διαβάζεις.

The core noun is βιβλία (neuter plural).
Therefore the verb βοηθάνε must be in 3rd person plural, to agree with that plural subject.

Structurally, think:

Όσα βιβλία διαβάζεις = the things that help you
σε βοηθάνε = (they) help you

So the underlying pattern is:

(Αυτά τα) βιβλία σε βοηθάνε πολύ
“(Those) books help you a lot”

The subordinate clause διαβάζεις is just telling you which books they are (the ones you read).

Why is it βοηθάνε and not βοηθούν? Are both correct?

Both βοηθάνε and βοηθούν(ε) are correct 3rd person plural present forms of βοηθάω / βοηθώ (“to help”).

βοηθάνε
- More colloquial / everyday
- Very common in speech
- Also used in informal writing
βοηθούν (or βοηθούνε)
- More standard / formal in writing (especially βοηθούν)
- βοηθούνε sounds more spoken/colloquial, like βοηθάνε

In this sentence, Όσα βιβλία διαβάζεις σε βοηθάνε πολύ, using βοηθάνε sounds natural, everyday, and completely correct.

Why do we use σε in σε βοηθάνε and not σου?

Σε and σου are both unstressed pronouns meaning “you”, but they have different cases / functions:

σε = accusative (direct object)
σου = genitive (often indirect object, possession, or “to you / for you” notions)

The verb βοηθάω takes its person being helped in the accusative in modern Greek:

Σε βοηθάω. = I help you.
Σε βοήθησε. = He/she helped you.
Σε βοηθάνε. = They help you.

So σε βοηθάνε is the correct pattern:
βοηθάνε (ποιον;) σε = they help (whom?) you.

Using σου here (σου βοηθάνε) would be ungrammatical.

What does πολύ add to the sentence? Why not πολλά?

Πολύ here is an adverb, modifying the verb βοηθάνε:

σε βοηθάνε πολύ = they help you a lot / very much.

Key distinction:

πολύ (adverb) → describes how much / to what extent something happens
- με βοηθάει πολύ = he/she helps me a lot
πολύς / πολλή / πολύ / πολλά (adjective/pronoun) → describes how many / how much of a noun
- πολλά βιβλία = many books

In this sentence, we are not saying “many books help you” but “the books help you a lot”. So:

Όσα βιβλία διαβάζεις σε βοηθάνε πολύ.
= However many books you read, they help you a lot.

Could I say Τα βιβλία που διαβάζεις σε βοηθάνε πολύ instead? What is the difference?

You can say:

Τα βιβλία που διαβάζεις σε βοηθάνε πολύ.

This is grammatically correct and natural. The difference is in nuance:

Τα βιβλία που διαβάζεις…
= “The books that you read…”
- Uses που, a simple relative pronoun.
- Just identifies which books we’re talking about.
Όσα βιβλία διαβάζεις…
= “As many books as you read / However many books you read…”
- Uses όσα, which carries an extra sense of quantity/extent.
- Implies all the books you read, whatever that number may be.

So Όσα βιβλία διαβάζεις σε βοηθάνε πολύ focuses more on the idea that every book you read contributes; the more you read, the more help you get.

Can I change the word order, like Σε βοηθάνε πολύ όσα βιβλία διαβάζεις?

Greek word order is fairly flexible, so several versions are possible:

Όσα βιβλία διαβάζεις σε βοηθάνε πολύ.
– Very natural, subject first.
Σε βοηθάνε πολύ όσα βιβλία διαβάζεις.
– Also grammatical and natural.
– Puts mild emphasis on σε βοηθάνε πολύ (“they help you a lot”), then specifies what helps you.
Πολύ σε βοηθάνε όσα βιβλία διαβάζεις.
– Also possible, with emphasis on πολύ (“a lot”).

What you normally wouldn’t say is something that breaks the units unnaturally, like splitting σε far away from βοηθάνε in a strange way. But all of the above are acceptable; word order mainly changes focus/emphasis, not basic meaning.

What tense/aspect is διαβάζεις here? Is it talking about now or a general habit?

Διαβάζεις is present tense, imperfective aspect, 2nd person singular of διαβάζω (“to read”).

In Greek, present imperfective commonly expresses:

Habitual / repeated actions
- Κάθε μέρα διαβάζεις βιβλία. = You read books every day.
General truths or regular behavior

In this sentence, Όσα βιβλία διαβάζεις σε βοηθάνε πολύ, it refers to your general habit of reading books, not just what you’re reading right this second.

If you wanted one specific past event, you would normally use the aorist:

Όσα βιβλία διάβασες σε βοήθησαν πολύ.
= The books (that) you read helped you a lot (on that occasion).

Could I use όσο instead of όσα? What is the difference?

You cannot just replace όσα with όσο here, because they agree with βιβλία:

βιβλία is neuter plural.
So the matching form is όσα (neuter plural), not όσο (neuter singular).

Compare:

Όσο βιβλίο διαβάζεις…
– Would sound wrong, because βιβλίο is normally countable and here we’re clearly talking about books in the plural.

Correct patterns:

Όσα βιβλία διαβάζεις… (plural, countable)
Όσο νερό πίνεις… (singular, uncountable: “as much water as you drink”)

So:

Use όσα with plural countable nouns like βιβλία.
Use όσο with singular mass/uncountable nouns, or singular nouns where that makes sense.

What is a good natural English translation, and what nuance should I keep in mind?

Two natural translations are:

“The more books you read, the more they help you.”
“However many books you read, they help you a lot.”

Nuance to notice:

Όσα suggests extent/quantity: it implicitly invites a “the more…, the more…” interpretation, even though Greek doesn’t explicitly repeat τόσο here.
The Greek sentence emphasizes that every book you read contributes to helping you; your reading (however much it is) is very beneficial.

So any English version that keeps the idea “your reading of books (in whatever amount) helps you a lot” captures the key nuance.