Learn Hardcore Greek: Η συμβουλή είναι πολύ σημαντική.

Η = The (feminine singular definite article)
συμβουλή = advice (literally: a piece of advice; feminine singular noun)
είναι = is (3rd person singular of είμαι = to be)
πολύ = very (adverb)
σημαντική = important (feminine singular form of the adjective)

So the full sentence is: The advice is very important.

Why do we say Η συμβουλή and not just συμβουλή, when in English we often say simply “Advice is very important”?

Greek normally uses the definite article (ο, η, το) much more than English, especially with:

Abstract nouns (love, health, advice, etc.)
General statements

So:

Η συμβουλή είναι πολύ σημαντική.
= Advice is very important. / The advice is very important.

If you say just Συμβουλή είναι πολύ σημαντική, it sounds odd or incomplete. The article Η is the natural, normal choice here, even though English might not use the.

Why is the article Η and not Ο or ΤΟ?

Because συμβουλή is a feminine noun. Each noun in Greek has a grammatical gender:

Masculine: ο φίλος (the friend, male)
Feminine: η γυναίκα (the woman)
Neuter: το βιβλίο (the book)

συμβουλή follows a common feminine pattern ending in -ή, so it takes the feminine article η:

η συμβουλή = the advice
της συμβουλής = of the advice (genitive)
τη(ν) συμβουλή = the advice (object / accusative)

How can I tell that συμβουλή is feminine just by looking at it?

Often (not always!) the ending gives you a strong clue. Many feminine nouns end in:

-η (e.g. η πόλη – the city, η στιγμή – the moment, η αρχή – the beginning)
-α (e.g. η θάλασσα – the sea, η χώρα – the country)

συμβουλή ends in -ή and behaves like these feminine nouns, so you learn it as:

η συμβουλή (feminine)

There are exceptions in Greek, so you should always learn new nouns together with their article: η συμβουλή, not just συμβουλή.

Why does σημαντική end in -ή? I thought “important” was σημαντικός.

The basic dictionary form of the adjective is:

σημαντικός (masculine)
σημαντική (feminine)
σημαντικό (neuter)

Adjectives in Greek must agree with the noun in:

Gender (masculine / feminine / neuter)
Number (singular / plural)
Case (nominative, etc.)

Here the noun is:

η συμβουλή → feminine, singular, nominative

So the adjective must also be:

feminine, singular, nominative → σημαντική

That is why we say:

Η συμβουλή είναι σημαντική.
The advice is important.

If the noun were masculine or neuter, you’d see:

Ο κανόνας είναι σημαντικός. (The rule is important.)
Το βιβλίο είναι σημαντικό. (The book is important.)

Why is it πολύ σημαντική and not πολλή σημαντική?

Here πολύ is an adverb meaning very. As an adverb, it is invariable: it does not change for gender, number or case.

πολύ σημαντική = very important (fem.)
πολύ σημαντικός = very important (masc.)
πολύ σημαντικό = very important (neut.)

The forms πολύς, πολλή, πολύ are adjectives/pronouns meaning much/many/a lot of, and they do change:

πολλή δουλειά = a lot of work (feminine noun)
πολύς κόσμος = many people (masculine)
πολλά βιβλία = many books (neuter plural)

In your sentence, we are not saying much important but very important, so the adverb form πολύ is correct.

Is συμβουλή like English “advice” (uncountable), or can it be counted in Greek?

In Greek, συμβουλή is a countable noun:

μία συμβουλή = one piece of advice
δύο συμβουλές = two pieces of advice
πολλές συμβουλές = many pieces of advice

So while English normally treats advice as uncountable (you don’t say two advices), Greek has a regular singular and plural:

Singular: η συμβουλή
Plural: οι συμβουλές

Example:

Μου έδωσε μια καλή συμβουλή.
He/She gave me a good piece of advice.

Your sentence talks about advice in general, so η συμβουλή there has a generic sense.

What is the role of είναι in this sentence, and can it ever be omitted like “am/is/are” sometimes are in other languages?

είναι is the 3rd person singular form of είμαι (to be), meaning is.

The main present‑tense forms of είμαι are:

(εγώ) είμαι – I am
(εσύ) είσαι – you are (singular)
(αυτός/αυτή/αυτό) είναι – he/she/it is
(εμείς) είμαστε – we are
(εσείς) είστε / είσαστε – you are (plural/polite)
(αυτοί/αυτές/αυτά) είναι – they are

In Η συμβουλή είναι πολύ σημαντική, είναι is essential; you cannot normally drop it in standard Greek. You can drop the subject pronoun (e.g. say Είναι πολύ σημαντική. without Η συμβουλή if the subject is clear from context), but the verb είναι stays.

Could I change the word order? For example, can I say Είναι πολύ σημαντική η συμβουλή?

Yes, Greek word order is more flexible than English. All of the following are grammatically correct:

Η συμβουλή είναι πολύ σημαντική.
Η συμβουλή είναι σημαντική, πολύ σημαντική. (more emphatic)
Είναι πολύ σημαντική η συμβουλή.

The most neutral and common version here is the original:
Η συμβουλή είναι πολύ σημαντική.

Putting η συμβουλή at the end (Είναι πολύ σημαντική η συμβουλή) can sound slightly more emphatic or contrastive, as if you are stressing that it is advice that is very important (perhaps in contrast to something else).

What case is η συμβουλή in here, and why?

η συμβουλή is in the nominative case, because it is the subject of the verb είναι.

For this noun you will most commonly see:

Nominative (subject): η συμβουλή – the advice
Genitive (of): της συμβουλής – of the advice
Accusative (object): τη(ν) συμβουλή – the advice (as an object)

In your sentence, Η συμβουλή is “what is important”, so it is the subject → nominative.

How do I pronounce Η συμβουλή είναι πολύ σημαντική?

Approximate pronunciation (in simple Latin letters) with stress marks:

Η συμβουλή → i simvouLÍ
είναι → Íne
πολύ → poLÍ
σημαντική → simandikÍ

So the whole sentence:

Η συμβουλή είναι πολύ σημαντική.
→ i simvouLÍ íne poLÍ simandikÍ

Key points:

η is pronounced like English ee in see.
The written accent (´) shows the stressed syllable:
- συμβουλή – stress on -λή
- είναι – stress on εί-
- πολύ – stress on -λύ
- σημαντική – stress on -κή
μπ, ντ, γκ, βδ etc. can be a bit tricky later, but here συμβουλή is just simvou-lee with a smooth v sound.