The "Subject + 是 + Noun" sentence

是(ㄕˋ; shì) is a linking verb. It connects a subject to additional information about the subject. 是 is equivalent to the English verb "to be," but their usages are not entirely the same:

1. Chinese does not conjugate verbs. So, there's no need to change the form of 是 according to the subject.

1. 是 doesn't connect subjects to adjectives. For example, the sentence "I am fine" cannot be directly translated as"我是好."(我 means l or me; 好(ㄏㄠˇ; hǎo) means fine) It doesn't make sense to a Mandarin speaker. You have to say "我很好." (很(ㄏㄣˇ; hěn) means very or quite; I'll make a lesson introducing its usage.)

Let's take a look at some examples of the "Subject + 是 + Noun" sentence.

• 我是美國人/我是美国人。(I am an American/I am American .)
  ㄨㄛˇ ㄕˋ ㄇㄟˇ ㄍㄨㄛˊ ㄖㄣˊ; wǒ shì měi guó rén 

美國(ㄇㄟˇ ㄍㄨㄛˊ; měi guó) means the  United States. 人(ㄖㄣˊ;rén) means person or people. In a previous lesson about personal pronouns, I have introduced the 亻(ㄖㄣˊ;rén) radical. The 亻 radical is derived from 人. That's why many words with the 亻 radical are related to people.
To say a person or people of a certain country, simply add 人 after the name of the country. For example, A Japanese is 日本(ㄖˋ ㄅㄣˇ; rì běn; Japan) + 人: 日本人.
Also, in English, we often have to put an article before a noun, such as "a" book or "an" American. But, there's no such rule in Mandarin. Even though 美國人 is a noun, there's no need to put anything before it. 

• 她們是學生。(They are students.)

學生 means student. 學 means learn.  生, as a verb, means to be born or to give birth. Here 生 means student or pupil.
Noticed that the subject 她們 is plural, while 學生 doesn't change to a plural form. That's because in Chinese nouns usually don't have plural forms, except we add 們 after pronouns or nouns related to humans. In the sentence 她們是學生, the plurality has already been indicated by 她們 so it would be redundant and awkward to add 們 after 學生.

• 贈品是毛巾。(The giveaways are towels.)

Like I said, most nouns in Chinese
don't have plural forms. A noun itself can be singular or plural, depending on the context.

• 甜點是蛋糕。(The dessert is cake. )

甜點 means dessert. 甜 means sweet. 點 means refreshment or a small amount of food in this phrase.
蛋糕 means cake. 蛋 means egg. 糕 is a general term referring any kind of food made of dough, baked, boiled or steamed.