Learn Hardcore Japanese: kinou ha tomodati ni hon wo agemasita.

In this sentence, 昨日 (yesterday) is marked with は to set it as the topic: “As for yesterday…”. Time expressions often take は when you want to frame the sentence around that point in time (e.g. contrast “yesterday” with other days).

Why is は pronounced wa instead of ha when it’s a particle?

Whenever は functions as the topic particle, its pronunciation changes from ha to wa. This is a historical spelling convention that applies only when it marks topics.

Why do we use the particle に after 友達?

に marks the indirect object or the recipient/beneficiary of an action. Here, “friend” is the person receiving the gift, so we say 友達に (“to a friend”).

Why is 本 followed by を?

を marks the direct object, i.e. what is being acted upon. Since you are giving 本 (a book), it takes を: 本を (“the book”).

Why isn’t there an explicit subject like “I” (私) in the sentence?

In Japanese, subjects are often omitted when they’re clear from context. Here the speaker is obviously the giver, so you don’t need to say 私は unless you want extra emphasis or contrast.

Why is the verb あげました used instead of くれました or もらいました?

あげる means “to give” from the speaker’s (or giver’s) perspective, so it’s used when you are the giver.
くれる means “(someone) gives to me” (receiver’s perspective).
もらう means “to receive.”
Since “I gave a book to a friend,” the correct choice is あげました in the polite past form.

What is the difference between the polite form あげました and the plain form あげた?

あげた is the plain (casual) past: you’d use it with friends or in writing/text among peers.
あげました is the polite past (ます-form), appropriate for formal situations or when speaking to someone you’re not close with.

How do you read 昨日? Are there other readings?

The everyday reading of 昨日 is きのう.
There is also the on’yomi (Chinese-derived) reading さくじつ, but that appears mainly in written news or very formal contexts.