Phrasal verbs are similar to idioms: take two separate words, put them together, and have a third new—and often unrelated—meaning.
verb + preposition = new meaning
Students often first learn a one-word verb, such as “to mention something”, or “to begin to talk about something”. A phrasal verb with a similar meaning is “to bring up”:
A teacher brings up the need for both formal and informal words.
A formal, one-word verb (mention) may be replaced with an informal, two-word verb (bring up). In the example above, we know that the teacher mentioned, or talked about, something.
This is entirely different from the verb “bring”:
Students bring a notepad and pen to class.
In this example, we know that the students have a notepad and a pen in class: before the students arrived, their notepads and pens were not in the room; now that the students are there, so are the pads and pens.