这不是橡皮。

This isn't rubber.

Source: Tatoeba

Sentence Composition

=1 这不 zhèbu (coll.) As a matter of fact, ... (used to introduce evidence for what one has just asserted)
=1 这不 zhèbù connects two clauses, emphasising how the following clause substantiates the preceding clause
=2 是 shì
  1. to be (followed by substantives only)
  2. correct; right; true
  3. (respectful acknowledgement of a command) very well
  4. (adverb for emphatic assertion)
=2 是 shì variant of 是 shì
3 橡皮 xiàngpí
  1. rubber
  2. an eraser

classifier: 塊