It's been raining since yesterday.

it's Contraction

— The contraction of "it is". Bring you coat. It's raining. "Where's the book?" "It's on the teacher's desk." "What's that?" "This? It's just a piece of paper." — The contraction of "it has". It's been a long time since I've had cake.

be Verb

— This verb tells us that a thing is present, or in a place. The book is on the table. There was someone in the room. Were you at the party? — Happen The party was on Saturday. The next meeting will be here. — Go somewhere; visit someone (only in the present perfect tense) I've never been to Disneyland. — A verb we use when describing. The book is blue. They were sad about losing the match. John is 35 years old this year. Im fine. He'd like to be a doctor. The film was terrible! The book is 5 euro. — Act like someone or something Look, Mummy! I'm being a dog! Woof, woof! — We use a form of be and a gerund-participle to make progressive verb forms. He is sitting on the chair. I've been waiting for half an hour. They will be leaving on Tuesday.

raining

— adjective falling in drops or as if falling like rain watched the raining apple blossoms

since Preposition

— From a particular time in the past until now or until a later time in the past. I haven't been there since 1969. Since you told me about it, I've been thinking a lot. — You use since to show that something known is the reason for something else. Since John can't be there, we'll try for another day. (The audience already knows that John can't be there.)

yesterday Pronoun

— Yesterday is the day before today. Yesterday, I went to the store. — Yesterday is the past in general. His clothes are like yesterdays fashion. I met her yesterday morning.

yesterday Noun

— Yesterdays are days in the past. He remembered all his yesterdays.