Why? because it changes according to each particular condition, for example,
- when you talk about your age, you use this verb to refer how old you are: I am 15 years old. ( Tengo 15 años)
- When want to express your state of mood, you say: I am really hungry ( en verdad tengo mucha hambre)
- when you want to talk about weather (climate) conditions, you say: It is cold ( I´m freezing) [Hace frío, me estoy congelando]
- when you describe someone, or even if you want to talk about what you are like, you say: I´m a sensitive and romantic guy [ soy un chico romántico y sensible]
- When we want to refer to cost or price, we say, how much is it? Oh! It is 5 dollars. ( ¿cuánto cuesta? ah, si! cuesta 5 dólares)
Now check the exercises and try to do them, but firs check the information pages or links to understand much better this "verb to be" topic.
