(1) You have asked a very important question. It even confuses some
of us native speakers such as I.
(2) Of course, this question cannot be answered in one liittle post.
(3) Basically, the traditional parts of speech are:
nouns (for example: apple)
verbs (for example: eat)
pronouns (for example: he)
adverbs (for example: carefully)
adjectives (for example: nice)
conjunctions (for example: because)
prepositions (for example: with)
interjections (for example: Oops!!!)
(4) Basically, a sentence has two parts: the subject + the predicate.
The girl with red hair loves to study English.
The subject = The girl with red hair.
The predicate = loves to study English.
There are also other parts of the sentence that you will have to
study, such as the object. (I love English. = English is the object.)
As you study these things in your book, please post your questions
here. The excellent teachers and many non-teachers (such as I)
will be delighted to answer you. Good luck.