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English as a Second Language/cant't or couldn't is used to expressnegative deduction

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Question
QUESTION: Dear Amy,

Is this tense sequence correct is the following sentence?

She can't have been at home because she would open the door when I knocked at it.

Best regards,
Antoine Ghannoum

ANSWER: Hello Antoine,

the sentence above is incorrect; it should read:

She couldn't have been at home because she would have opened the door when I knocked at it.


We can use 'can have' to speculate about what has happened but only in questions and negative sentences and with words such as 'hardly', 'never' and 'only'.

Can she have forgotten about our meeting?
He can't have seen us.
They can hardly have thought that I was not interested in the job.

See more here:

http://www.englishgrammarsecrets.com/couldhave/menu.php

I hope this helps.

Best,

Amy

---------- FOLLOW-UP ----------

QUESTION: Please let me know:
1. What is the type of the above sentence? Is it conditional?
2. Why did you change "can't" into "couldn't"? I rechecked my grammar book and it says "Negative deduction about a past event is expressed by "can't/couldn't" + perfect infinitive of any verb".
Looking forward to hearing from you.

Answer
Hello again Antoine,

1. What is the type of the above sentence? Is it conditional?

She couldn't have been at home because she would have opened the door when I had knocked at it.

Is this the sentence you're asking about Antoine? If so yes the second part of this sentence is a conditional sentence of the third type.

There is also a category of clauses called "because clauses" you can read more about them here: http://www.grammar-quizzes.com/8-15.html

2. Why did you change "can't" into "couldn't"? I rechecked my grammar book and it says "Negative deduction about a past event is expressed by "can't/couldn't" + perfect infinitive of any verb".

I changed can't into couldn't because the third conditional sentences form is the following:

if/when + Past Perfect Tense (conditional clause)   would have + past participle (main clause)
when I had knocked at it (Conditional clause)  she would have opened the door (main clause)


The verb in the first clause: "she couldn't have been at home" has to follow the verb sequence in the main sentence of the conditional clause, you can't use a present tense verb here when the verb in the main clause of the conditional is a past conditional one. The two verbs have to match:

couldn't have been .... would have opened.

Please realize, once again that the rule you have quoted from that grammar book is out of context. Moreover, English grammar is not set in stone; there is a lot of flexibility in its rules.

I hope this helps.

Best,

Amy

English as a Second Language

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Amy Baker

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I can answer question about grammar, spelling, syntax, idioms, reading and/or writing that pertain to English as a Second Language. I am knowledgeable about both TOEFL and IELTS.

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I'm a certified ESL teacher with 12 years of experience teaching K-12 and adults.

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BSc MEd TESL post grad program for k-12 TESL post grad program for adult ed

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