The で particle, では and じゃ

Normally, で marks the "circumstance" or "means" behind an action, or the “location” where such action occurs.

剣で出来ている
It is made of swords

刀で殺した
He killed with a katana

バスで来た
I came by bus

学校で働く
To work in a school

で originally comes from contracting the compound particle にて. You may still see にて in old fashioned or formal writing/announcements.

で can also be used like it's the て form of だ. In this usage, it assumes the typical conjunction usage of て form as with verbs or adjectives, except it works with nouns or noun-like words.

中学生で一人暮らしはできない
To be a middle schooler and live alone is not possible

Sometimes distinguishing between the case marker で particle (location, means, etc) and the てform of だ conjunction is not always clear. What matters, however, is that the meaning of the sentence should still be clear. You shouldn’t worry about pinpointing the exact dictionary definition for each particle you see used in a sentence. If you understand the meaning, that’s good enough.

This brings us to では. では is a compound particle. The は here can be considered the topic marker, but the meaning of では together is more than just the sum of its parts. では often marks the circumstances or condition of another state.

公園ではアイスが売られる
Ice cream is sold at the park.

This also extends to the state of being, as we already saw in lesson 1.

私は学生ではない
I am not a student.

As a reminder, では is often contracted as じゃ. Some dialects may use different copulas from だ. Examples are や (often used in kansai dialect) and じゃ (often used by old men in fictional media to sound old fashioned). This じゃ is a contraction of something other than では. The usual では is not a copula.

ペンじゃ
Tis a pen. (it's a pen.)