Why can't I use that instead of which?

[Saadi]

English Error, Elementary Level

ESL/EFL Test #5 "Relative Pronouns", question 10

The grass, whom I cut every week, seems to grow very quickly.

(a) The
(b) whom
(c) seems

English Error, Elementary Level

ESL/EFL Test #5 "Relative Pronouns", answer 10

The grass, which I cut every week, seems to grow very quickly.

Correct entry: which
The error was: (b) whom

You have found the error but your entry is incorrect.
The grass, That I cut every week, seems to grow very quickly.
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Why can't I use that instead of which?

Saadi

[Alan]

Hi,

In the test sentence you will notice there is a comma after the word 'grass'. This means that the sentence 'I cut every week' is not a defining relative clause but a non-defining one. You cannot use the comma before the relative conjunction 'that'.

Alan
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English as a Second Language
You can read my ESL story Reflections

[Conchita]

This site has three lessons on relative pronouns here:

ESL Lesson: Relative Pronoun
http://www.english-test.net/lessons/21/index.html