Question 1

I'm a bit confused. Non-essential phrases (the stuff bound by commas) can be removed and the sentence should still make sense right?

Then why is it that in the second practice question- for which the answer is "While they have been largely replaced by Arabic numerals, Roman numerals are still used in a few _contexts because_ it is sometimes useful to have distinct numerals for distinct purposes, like when numbering the quadrants of a graph."- when we remove the words bound by commas ("Roman...purposes") the sentence doesn't make sense?
Is it that when a conjunction is added in a phrase bound by commas it no longer behaves like a nonessential phrase? Or do we consider the words on both sides of the conjunction as two separate phrases?

Also, I don't completely understand these terms- phrase, clause and non-essential phrase. It'd be great if someone could explain them using examples.

Accepted Answer

What you have between the commas in that sentence, "Roman numerals are still used in a few contexts because it is sometimes useful to have distinct numerals for distinct purposes" functions as an entire sentence by itself. It's made up of an independent clause ("Roman numerals are still used in a few contexts") and a dependent clause ("because it is sometimes useful to have distinct numerals for distinct purposes").
Clauses are bits of words with a subject and a verb in them. Take "Roman numerals" and "are used" in the first clause, and "it" and "is" in the second clause. This is the key difference between them and phrases. A clause, even a dependent one, will always have a subject and a verb, while a phrase will never have both a subject and a verb in it. 
The way I've learned it is that a phrase is just a group of words acting together to express one idea, like in "around the world" or "slurping greedily". 
Now, nonessential phrases are phrases that can be removed from the sentence, like you said. The main way we signify that a phrase is nonessential is with the commas surrounding it. In the sentence: "Usnavi, a very hairy man, was fearsome to behold", knowing that Usnavi is hairy isn't essential to getting at the main meaning of the sentence, which is that he's fearsome. For the most part, you can tell if a phrase is nonessential or not just from its meaning and how it relates to the meaning of the sentence. 
There are tricky situations, though. Take the sentence: "The person cutting Usnavi's hair was even hairier than he was." Here, we don't put commas around "cutting Usnavi's hair" because it's essential information to understanding who the man is. It's a bit confusing. 
So a phrase is a group of words that work together to achieve a greater meaning, a clause is a group of words with a subject and a verb, and a nonessential phrase is a phrase is separated from the rest of the sentence by commas because without it, the sentence is still specific and has a meaning that makes sense. You can have nonessential clauses as well, and just plain words too, which you also have to put commas around.

Question 2

how do you properly know when it's a good time to use commas and semicolons ?

Accepted Answer

Semicolons

-separate elements in a series *if* the elements already contain commas. (On my trip I visited Tallahassee, Florida; Atlanta, Georgia; and Montgomery, Alabama.)

-connect related independent clauses - which are full sentences - *if* there is no conjunction like "and" joining them. (My class is long; it is also boring.)

-are used between independent clauses that are connected with a transitional phrase like "as a result." (I did not stay up late to study for my exam; as a result, I failed the class.)

-are used in between independent clauses if they are really long or if they are already punctuated with commas. (I ordered vanilla, moosetracks, and strawberry ice cream from the shop; their most famous ice cream, however, is chocolate.)

Commas

-separate elements in a series *if* there are not already commas within the elements. (On my trip I visited Florida, Georgia, and Alabama.)

-join independent clauses *if* there is a conjunction to link them. (My class is long, and it is also boring.)

-set apart introductory phrases or words. (While he was eating lunch, he spilled his coffee.)

-set apart nonessential phrases or words. (My aunt, who has the long blonde hair, is standing next to my uncle.) - If you can take this phrase out without interrupting the flow of the sentence, it is nonessential.

-are used between coordinate adjectives, which are adjectives with "equal status" in describing their noun - neither is subordinate to the other. (The cold, dark cave was home to many creatures.) - If you can place an "and" between them, and it still makes sense, then they are coordinate adjectives and should have a comma between them. You would *not* put a comma between little and old in "the little old lady."

-should always go after the last element before the "and" in a string of elements. This is called the Oxford comma. (Texas, New York, West Virginia, and Montana are states.) You would *not* write "Texas, New York, West Virginia and Montana are states."

While this is certainly not an exhaustive list of rules, they are usually the most helpful, and they also show the contrast between commas and semicolons. Sorry for such a long reply but hope that it helps!

Question 3

Regarding the example used in the Sentence Fragments exercise, why are the words "that are" omitted between "publications" and "released"?

"Periodical" is an umbrella term used to refer to various publications released on a regular schedule, including magazines, academic journals, and yearbooks.

Accepted Answer

The sentence is grammatically correct whether it's written as

"Periodical" is an umbrella term used to refer to various publications released on a regular schedule, including magazines, academic journals, and yearbooks.

or

"Periodical" is an umbrella term that is used to refer to various publications that are released on a regular schedule, including magazines, academic journals, and yearbooks.

The first option is just shorter, so it's "correct"

Question 4

Why is "He tried to lose weight" a dependent clause?

Accepted Answer

It shouldn't be, did you see it on this article? "He tried to lose weight" is a complete thought that stands on its own, so its an independent clause. If you add a subordinating conjunction to the front like "because he tried to lose weight", that wouldn't be a complete thought by itself and so would be a dependent clause.

SAT

Course: SAT > Unit 5

Sentence fragments | Quick guide

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