Used like 'this' in English, but for now we will use it only by itself and not with nouns. We will say things like, "This is my book." and not "This book is mine."

The verb 是 joins two noun phrases, identifying a relationship between the two.

Don't be tempted to use 是 with an adjective in a descriptive sentence. Adjectives in Mandarin are full predicates, and as such do not require the verb 'to be'. Remember we call them stative verbs or adjectival predicates.

Use 不是 to negate a 是 sentence.

這是 is a simple way to identify a person or thing, or to introduce someone.