Learn Hardcore Greek: Θεωρείται ότι η ανάγνωση βιβλίων είναι πολύ αποτελεσματική για τη μνήμη. - It is considered that reading books is very effective for the memory.

It is the 3rd person singular, present tense, passive voice of the verb θεωρώ (to consider).
θεωρώ (active) = I consider
θεωρείται (passive) = it is considered

So Θεωρείται ότι… literally means “It is considered that…”, just like English “It is considered that reading books is very effective for memory.”

Why is there no subject before Θεωρείται? Who is doing the considering?

In Greek, Θεωρείται is used impersonally, similar to English “It is considered” where “it” doesn’t refer to anything concrete.

There is no explicit subject like “people” or “they”.
The idea is: “People in general consider that…”, but Greek leaves that subject understood and uses the passive verb alone.

You could make the subject explicit with the active voice:

Πολλοί θεωρούν ότι η ανάγνωση βιβλίων είναι πολύ αποτελεσματική για τη μνήμη.
= Many people consider that reading books is very effective for memory.

But the original sentence deliberately keeps it impersonal and general: “It is considered that…”

What is the role of ότι in this sentence? Is it the same as English “that”?

Yes. Here ότι is a conjunction meaning “that”, introducing a clause:

Θεωρείται ότι η ανάγνωση βιβλίων είναι…
= It is considered *that reading books is…*

A couple of useful distinctions:

ότι vs πως
- In this use, ότι and πως both mean “that” and are usually interchangeable:
  - Θεωρείται ότι…
  - Θεωρείται πως…
- ότι is a bit more neutral/formal; πως can feel a bit more colloquial.
ότι vs ό,τι
- ότι (no comma) = “that” (conjunction)
- ό,τι (with comma) = “whatever / anything that” (pronoun)
  In our sentence it must be ότι (no comma), because it introduces a statement: that reading books is…

Why is it η ανάγνωση and not just ανάγνωση? Do we have to use the article here?

Greek uses the definite article much more than English, especially with abstract nouns and activities.

η ανάγνωση βιβλίων = “the reading of books” (i.e. the activity in general)

In English we normally say simply “reading books”, without “the”, but in Greek, saying just ανάγνωση βιβλίων without the article sounds incomplete or unnatural in this context.

So:

η ανάγνωση βιβλίων είναι πολύ αποτελεσματική…
≈ The reading of books is very effective…
(which in normal English becomes Reading books is very effective…)

In general, when you turn a verb into a noun to talk about the activity in general (το διάβασμα, η ανάγνωση, η μελέτη), you almost always use the article:

Το διάβασμα βιβλίων βοηθάει.
Η μελέτη ξένων γλωσσών είναι χρήσιμη.

Why is it ανάγνωση βιβλίων and not ανάγνωση βιβλία?

Because βιβλίων is in the genitive plural, which is very common after certain nouns to mean “the X of Y”.

η ανάγνωση = the reading
η ανάγνωση βιβλίων = “the reading of books”

So structurally it’s like:

the reading of books → η ανάγνωση βιβλίων

If you wanted to avoid the genitive, you’d normally switch back to a verb:

Το να διαβάζεις βιβλία είναι πολύ αποτελεσματικό για τη μνήμη.
= Reading books (to read books) is very effective for memory.

But once you choose the noun η ανάγνωση, Greek prefers to link what is read in the genitive: βιβλίων.

Why is αποτελεσματική feminine? What does it agree with?

αποτελεσματική is an adjective meaning “effective”.

Adjectives in Greek agree with the noun they describe in gender, number, and case.

Here:

η ανάγνωση = feminine, singular, nominative
So the adjective must also be feminine, singular, nominative:
- πολύ αποτελεσματική

That’s why it’s:

η ανάγνωση βιβλίων είναι πολύ αποτελεσματική
not, for example, πολύ αποτελεσματικό (neuter) or πολύ αποτελεσματικός (masculine).

If the subject were neuter plural, e.g. τα βιβλία, you would say:

Τα βιβλία είναι πολύ αποτελεσματικά για τη μνήμη.

Why is πολύ not changing form here? Shouldn’t it be πολλή (feminine)?

πολύ can be either an adjective or an adverb, and the form changes depending on the role:

As an adjective (meaning “a lot of / many”), it agrees with the noun:
- πολύς χρόνος (masc.) = a lot of time
- πολλή δουλειά (fem.) = a lot of work
- πολλά βιβλία (neut. pl.) = many books
As an adverb (meaning “very” or “a lot”), it is invariable:
- πολύ αποτελεσματική = very effective (modifies the adjective)
- δουλεύει πολύ = he/she works a lot (modifies the verb)

In our sentence, πολύ modifies the adjective αποτελεσματική (how effective?), so it is an adverb, and it stays πολύ, without changing form.

Why do we say για τη μνήμη and not just για μνήμη? Do we always need the article with μνήμη?

Greek often uses the definite article with abstract nouns when talking about them in general.

για τη μνήμη here means “for (the) memory in general”, not a specific memory of one event.

In English we say:

…is very effective *for memory.*

But Greek prefers:

…είναι πολύ αποτελεσματική για τη μνήμη.

Compare:

Η μουσική είναι καλή για την υγεία.
= Music is good for (the) health.
Η άσκηση είναι σημαντική για τον εγκέφαλο.
= Exercise is important for (the) brain.

Using για μνήμη without the article would usually sound incomplete or odd in this generic sense. The article τη here does not mean “for one specific memory”; it marks the concept in general.

Why is it τη μνήμη and not την μνήμη?

Both τη μνήμη and την μνήμη are pronounced the same in modern Greek. The difference is mostly a matter of spelling conventions.

Traditional modern spelling uses:

την before vowels and certain consonants (κ, π, τ, ξ, ψ, μπ, ντ, γκ, τσ, τζ)
τη before other consonants

Since μ is not in that “special” group, standard spelling is:

τη μνήμη

However:

Many modern writers just use την everywhere in informal writing.
You will see both τη μνήμη and την μνήμη; τη μνήμη follows the more “school-book” rule.

Could we change the word order and say: Η ανάγνωση βιβλίων θεωρείται πολύ αποτελεσματική για τη μνήμη? Is that the same?

Yes, that sentence is also correct and natural, and very common:

Η ανάγνωση βιβλίων θεωρείται πολύ αποτελεσματική για τη μνήμη.
= Reading books is considered very effective for memory.

Differences in structure:

Original:
- Θεωρείται ότι η ανάγνωση βιβλίων είναι πολύ αποτελεσματική…
- Impersonal “It is considered that…” construction.
Alternative:
- Η ανάγνωση βιβλίων θεωρείται πολύ αποτελεσματική…
- The subject η ανάγνωση βιβλίων comes first and is directly linked to θεωρείται:
  “Reading books is considered very effective…”

Meaning is essentially the same; the second version may feel a bit more straightforward and is very natural stylistically.

Could we say Πιστεύεται ότι… or Λέγεται ότι… instead of Θεωρείται ότι…? Would it change the meaning?

You can, but there is a slight nuance difference:

Θεωρείται ότι…
= It is considered that…
→ often sounds a bit more evaluative/judgment-based (an assessment).
Πιστεύεται ότι…
= It is believed that…
→ emphasizes belief/opinion, sometimes without strong evidence.
Λέγεται ότι…
= It is said that…
→ highlights rumor, hearsay, or what people say.

In this context:

Θεωρείται ότι η ανάγνωση βιβλίων είναι πολύ αποτελεσματική για τη μνήμη.
suggests a generally accepted evaluation.

Using Πιστεύεται or Λέγεται would shift the tone slightly towards belief or what people say, but in everyday use, all three can often be translated as “It is thought that…” or “It is said that…”.

How would I say “was considered” instead of “is considered” in this type of sentence?

You need the past tense of the passive verb θεωρούμαι.

Common options:

Imperfect (past ongoing/habitual):
- Θεωρούνταν ότι η ανάγνωση βιβλίων ήταν πολύ αποτελεσματική…
  = It was (used to be) considered that reading books was very effective…
Aorist (past simple, one-time event):
- Θεωρήθηκε ότι η ανάγνωση βιβλίων ήταν πολύ αποτελεσματική…
  = It was considered (at some point, in some instance) that reading books was very effective…

In many contexts, Θεωρούνταν will be the natural choice for a general past state:

Παλιά, θεωρούνταν ότι η ανάγνωση βιβλίων ήταν πολύ αποτελεσματική για τη μνήμη.
= In the past, it was considered that reading books was very effective for memory.