A nominal sentence in Arabic starts with a noun.  This type of sentences can be deconstructed into two parts. The Subject (mubtadaɁ) as its first part and the second one predicate (xabar). It is better for the learner of Arabic to keep in mind this piece of information which can develope their  sense of Arabic. Whenever you start a sentence with a noun, all the information you include later in the sentence must relate that noun (subject).
No linking verb is required between the two parts. Therefore, in translation, we notice that (is, am and are) are not used in Arabic. In this lesson, we will study the two parts of the nominal sentence, namely mubtadaɁ and xhabar . Here are some simple examples to start with:
MubtadaɁ is originally taken from the three-radical root  daɁda ( بدأ) which literally means (he started). The word (mubtadaɁ) means the thing or person you start talking about. Have a look at the diagram below and see what types of words the mubtadaɁ can be.
The subject mubtadaɁ, then, can be:
a noun ( proper or common)
a combination of a noun and a pronoun (possessive adjectives my, his etc. are the English equivalents),
a combination of two nouns (idhafa).and
a pronoun.
The MubtadaɁ can be a proper noun, i.e. a a word that names people, languages, places, etc.
Because the subject (mubtata) must always be definite it should in this case be prefixed with the definite article (ال) and therefore, it cannot be entailed with nunation.
The pronouns (nominative) can be the subject (mubtadaɁ) of a nominal sentence. To know more about the nominative pronouns check this.

Idhafa means (addition), a nound is being added to another noun to show the relation between both nouns. It is similar to possessive case in English. For detailed explanation of idhafa check this link.
Here is a detailed example of how the subject (mubtadaɁ) can be defined by the following noun; that is  (idahfa)

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