This article will give you an overview of the key notions in Greek grammar to give you an idea of what you should focus on learning.
The Greek noun system displays inflection for two numbers (singular and plural), three genders (masculine, feminine and neuter), and four cases (nominative, genitive, accusative, and vocative).
As in many other Indo-European languages, the distribution of grammatical gender across nouns is largely arbitrary and need not coincide with natural sex.
Case, number and gender are marked on the noun as well as on articles and adjectives modifying it.
While there are four cases, they are very often represented by identical forms. Only one sub-group of the masculine nouns actually has four distinct forms in the four cases.
There are two articles in Modern Greek, the definite and the indefinite. They are both inflected for gender and case, and the definite article also for number. The article agrees with the noun it modifies.
The definite article is used frequently in Greek, such as before proper names and nouns used in an abstract sense. For example,
The indefinite article is identical to the numeral one and has only singular. The use of the indefinite article is not dictated by rules and the speaker can use it according to the circumstances of their speech. Indefiniteness in plural nouns is expressed by the bare noun without an article. For example,
However, the indefinite article is not used in Greek as often as in English because it specifically expresses the concept of "one".
Adjectives agree with nouns in gender, case, and number. Therefore, each adjective has a threefold declension paradigm for the three genders.
Adjectives show agreement both when they are used as attributes, e.g. η όμορφη γυναίκα (i omorfi gynaika, "the beautiful woman") and when they are used as predicates e.g. η γυναίκα είναι όμορφη (i gynaika einai omorfi, "the woman is beautiful").
Adjectives in Modern Greek can form a comparative for expressing comparisons. Similar to English, it can be formed in two ways, as a periphrastic form (as in English beautiful, more beautiful) and as a synthetic form using suffixes, as in English tall, tall-er.
While in English adverbs are usually formed by adding -ly to adjectives, in Greek many adverbs are formed from adjectives, simply by changing the suffix -ος of the singular masculine form of adjectives to -α. Examples:
Other adverbs maintain the Ancient Greek suffix –ως, for example: ακριβώς (exactly) εντελώς (totally).
Greek pronouns include personal pronouns, reflexive pronouns, demonstrative pronouns, interrogative pronouns, possessive pronouns, intensive pronouns, relative pronouns, and indefinite pronouns.
Greek personal pronouns have three cases: nominative, genitive, accusative.
The Greek verb can take many different forms which may indicate five properties: person, number, voice, tense and mood.
Person is the verb form that expresses the speaker (1st person), the person addressed (2nd person) or the person, animal or thing spoken of (3rd person).
Number is the verb form that shows the singularity or the plurality of the subject of the verb.
The Greek verb has two numbers, the singular and the plural, and three persons in each number as the English verb but unlike the English verb, the person and the number in the Greek verb are included in the ending.
Each person either in singular or in plural has a clearly distinguished ending. Therefore, the use of personal pronouns before the verbs are not obligatory in Greek. Nevertheless, personal pronouns may be used for emphatic reasons. The second person plural is also used as the polite form.
Greek verb morphology is mostly structured around a basic 2-by-2 contrast of two aspects, namely imperfective and perfective, and two tenses, namely past and non-past (or present).
The aspects are expressed by two separate verb stems, while the tenses are marked mainly by different sets of endings. There are also two imperatives, one for each aspect.
In addition to these basic forms, Greek also has several periphrastic verb constructions. All the basic forms can be combined with the future particle θα (historically a contraction of θέλει να, 'want to').
Combined with the non-past forms, this creates an imperfective and a perfective future. Combined with the imperfective past it is used as a conditional, and with the perfective past as an inferential.
There is also a perfect, which is expressed with an inflected form of the auxiliary verb έχω ('have'). It occurs both as a past perfect (pluperfect) and as a present perfect.
In Modern Greek, prepositions normally require the accusative case: από (from), για (for), με (with), μετά (after), χωρίς (without), ως (as) and σε (to, in or at).
The preposition σε, when followed by a definite article, fuses with it into forms like στο (σε + το) and στη (σε + τη).
While there is only a relatively small number of simple prepositions, the two most basic prepositions σε and από can enter into a large number of combinations with preceding adverbs to form new compound prepositions, for example, πάνω σε (on), κάτω από (underneath), πλάι σε (beside).
A few prepositions that take cases other than the accusative have been borrowed from Ancient Greek κατά (against), υπέρ (in favor of, for), αντί (instead of). Other prepositions live on in a fossilized form in certain fixed expressions (for example, εν τω μεταξύ 'in the meantime', dative).
This is a very brief overview of Greek grammar. To truly master it, you will need to study each of the parts of speech in much more detail. However, this overview will hopefully give you a general idea of the Greek grammatical system and of the main points you should consider when learning.