kono sen ha nagai.

It is read:

この せん は ながい。
Romanized: kono sen wa nagai.

A natural English translation is This line is long.

この means this when it comes directly before a noun.

So:

• この 線 = this line

A useful comparison:

• これ = this one / this thing
• この + noun = this _

So you say:

• これ は 長い。 = This is long.
• この 線 は 長い。 = This line is long.

線 means line and is read せん in this sentence.

So:

• この 線 = this line

This kanji is also used in words like:

• 電車線 kinds of rail-related compounds
• 直線 = straight line

But by itself here, 線 simply means line.

は marks the topic of the sentence.

Here, この線は means something like:

• As for this line, ...
• This line, ...

Then 長い gives the comment about that topic:

• この線は長い。 = As for this line, it is long.

In beginner-friendly English, it is usually translated simply as This line is long.

That is a special pronunciation rule for the particle は.

• As a normal syllable, は is usually read ha
• But when it is the topic particle, it is pronounced wa

So in this sentence:

• written: は
• pronounced: wa

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:

• 長い = long / is long

So:

• この線は長い。 literally works as This line [topic] long

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:

• 長い = long
• この線は長い。 = This line is long.

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:

• 背が高い = tall (for a person)
• 高い = high / tall
• 長い = long

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:

• この線は長い。

Yes. この線は長いです。 is the polite version.

Compare:

• この線は長い。 = plain style
• この線は長いです。 = polite style

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:

• topic + comment

So:

• この線は = as for this line
• 長い = is long

That gives:

• この線は長い。

English usually says This line is long, but Japanese builds the sentence differently.

Sometimes, yes, but the nuance changes.

• この線は長い。 = This line is long / as for this line, it is long
• この線が長い。 = This line is the long one / this line is the one that is long

Very roughly:

• は presents this line as the topic
• が can give it more focus or emphasis

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:

• 長い線 = a long line
• この長い線 = this long line

But in your sentence:

• この線は長い。

the adjective is at the end because it is describing the topic as the main statement: This line is long.