Japanese nouns have no grammatical number, gender or article aspect. The noun hon (本) may refer to a single book or several books; hito (人) can mean "person" or "people", and ki (木) can be "tree" or "trees".
Where number is important, it can be indicated by providing a quantity (often with a counter word) or (rarely) by adding a suffix or sometimes by duplication.
However, as part of the extensive pair of grammatical systems that Japanese possesses for honorification and politeness, nouns too can be modified. Nouns take politeness prefixes (which have not been regarded as inflections): o- for native nouns and go- for Sino-Japanese nouns.
Grammatical cases in Japanese are marked by particles placed after the nouns. A distinctive feature of the Japanese language is the presence of two cases which are roughly equivalent to the nominative case in other languages: one representing the sentence topic, other representing the subject.
In Japanese, nouns and verbs can modify nouns, with nouns taking the の particles when functioning attributively (in the genitive case) and verbs in the attributive form. These are considered separate classes of words, however.
Most of the words that can be considered to be adjectives in Japanese fall into one of two categories: variants of verbs and variants of nouns:
Adjectival verbs end with い i (but never えい ei) in base form. They may predicate sentences and inflect for present and past, positive and negative.
Adjectival nouns (na-adjectives) are inflected by dropping the -na and replacing it with the appropriate form of the verb da, the copula. They also inflect for present and past, positive and negative. As with adjectival verbs, adjectival nouns are also made more polite by the use of です desu.
Adverbs in Japanese are not as tightly integrated into the morphology as in many other languages. Indeed, adverbs are not an independent class of words, but rather a role played by other words. For example, every adjective in the continuative form can be used as an adverb; thus, 弱い yowai 'weak' (adj) → 弱く yowaku 'weakly' (adv). The primary distinguishing characteristic of adverbs is that they cannot occur in a predicate position, just as it is in English.
Japanese has a large number of pronouns, differing in use by formality, gender, age, and relative social status of speaker and audience.
Further, pronouns are an open class, with existing nouns being used as new pronouns with some frequency. This is ongoing; a recent example is jibun (自分, self), which is now used by some young men as a casual first-person pronoun.
Pronouns are used less frequently in the Japanese language than in many other languages, mainly because there is no grammatical requirement to include the subject in a sentence.
The common English personal pronouns, such as "I", "you", and "they", have no other meanings or connotations. However, most Japanese personal pronouns do. Consider for example two words corresponding to the English pronoun "I": 私 (watashi) also means "private" or "personal". 僕 (boku) carries a masculine impression; it is typically used by males, especially those in their youth.
Japanese words that refer to other people are part of the encompassing system of honorific speech and should be understood within that context. Pronoun choice depends on the speaker's social status (as compared to the listener's) as well as the sentence's subjects and objects.
Verbs in Japanese are rigidly constrained to the ends of clauses in what is known as the predicate position. This means that the verb is always located at the end of a sentence.
The subject and objects of the verb are indicated using particles, and the grammatical functions of the verb — primarily tense and voice — are indicated by means of conjugation.
Verbs in Japanese are conjugated to show tenses, of which there are two: past and present (or non-past) which is used for the present and the future.
Verbs have two tenses indicated by conjugation, past and non-past. The semantic difference between present and future is not indicated through conjugation. Usually, there is no ambiguity as the context makes it clear whether the speaker is referring to the present or future.
Voice and aspect are also indicated by means of conjugation, and possibly agglutinating auxiliary verbs. For example, the continuative aspect is formed by means of the continuative conjugation known as the gerundive or -te form, and the auxiliary verb iru "to be»:
見る miru (to see) - 見ている mite iru (to be seeing)
Japanese prepositions function differently from English and other Indo-European languages. Japanese words that serve the same function as the English preposition often follow the noun phrase they modify, that is most of them are not prepositions but postpositions.
Furthermore, the role of the preposition is often split between multiple words, one or more of which is a standard Japanese particle. Here is an example with the preposition ‘above’.
山の上に雲が見えた。 - We saw clouds above the mountain (山).
When describing something that is physically above something else, the noun 上 is used. Since it is a noun, it is linked to other nouns using the particle の. Since the resulting item describes a location, the particle に as a location marker may be required.
The Japanese language has many honorifics, referred to as keigo (literally "respectful language"), parts of speech that show respect. Their use is mandatory in many social situations. Honorifics in Japanese may be used to emphasize social distance or disparity in rank or to emphasize social intimacy or similarity in rank.
The system is very extensive, having its own special vocabulary and grammatical forms to express various levels of respectful, humble, and polite speech. It closely resembles other honorifics systems found in the East Asian cultural sphere, such as the one used in Korean.
This is a very brief overview of Japanese grammar. To truly master it, you will need to study each of the parts of speech and grammar aspects in much more detail. However, this overview will hopefully give you a general idea of the Japanese grammatical system and of the main points you should consider when learning.