この means this when it comes directly before a noun.
So:
A useful comparison:
So you say:
That is a special pronunciation rule for the particle は.
So in this sentence:
That is why the sentence is read kono sen wa nagai.
In Japanese, you often do not need a separate word like is.
Here, 長い is an i-adjective, and i-adjectives can end a sentence by themselves:
So:
Natural English adds is, but Japanese does not need it here.
長い is an i-adjective.
That means it is an adjective ending in -い, and it can act like the predicate of the sentence.
So:
This is different from English, where adjectives usually need is/are in a sentence like this.
長い usually means long.
With 線 (line), it definitely means long, because a line has length.
For people or buildings, Japanese usually uses other words instead:
So in this sentence, 長い means long, not tall.
No. Normally, Japanese is written without spaces:
この線は長い。
Spaces are often added in textbooks or beginner materials to make the parts easier to see.
So both are the same sentence, but the normal written form is:
Yes. この線は長いです。 is the polite version.
Compare:
Both mean This line is long.
The version without です is not wrong; it is just less polite and more plain/direct.
Japanese basic sentence order is often:
So:
That gives:
English usually says This line is long, but Japanese builds the sentence differently.
Sometimes, yes, but the nuance changes.
Very roughly:
For a simple neutral statement, は is very natural here.
Yes, if you want to say a long line instead of this line is long.
In Japanese, adjectives come before the noun they modify:
But in your sentence:
the adjective is at the end because it is describing the topic as the main statement: This line is long.