这不是猫，这是狗。

This is not a cat, it is a dog.

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 猫 māo
  1. (dialect) to hide oneself
  2. (loanword) (coll.) modem

classifier: 隻
=3 猫 máo used in 猫腰 máoyāo
=4 这 zhè
  1. (pronoun) this; these
  2. (bound form) this; the (followed by a noun)
  3. (bound form) this; these (followed by a classifier) (in this sense, commonly pr. zhèi, esp. in Beijing)
=4 这 zhèi (coll.) this
=5 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)
=5 是 shì variant of 是 shì
6 狗 gǒu dog

classifiers: 隻, 條