Learn Russian: Книга скучная, поэтому мы читаем новый журнал. - The book is boring, so we are reading a new magazine.

Both words connect causes and results, but they go in different directions. Потому что means "because" and gives the reason. Поэтому means "so" or "that's why" and gives the result. Here, the book being boring is the reason, and reading the magazine is the result, so we use поэтому.

Where is the word for "is" in the first part of the sentence?

In Russian, the present tense of the verb "to be" is almost always left out. You just put the subject (книга) right next to its adjective (скучная) to say "the book is boring."

Why are the adjectives скучная and новый spelled with different endings?

Adjectives must match the gender of the noun they describe. Книга is a feminine noun, so it takes the feminine adjective ending -ая (скучная). Журнал is a masculine noun, so it takes the masculine ending -ый (новый).

Since we are reading the magazine, shouldn't новый журнал be in the accusative (object) case? Why does it look like the dictionary form?