Apostrophe

[Casiopea]

1. is incorrect. Try, six months (of) credit on electrical goods.
2. is incorrect. A plural noun is functioning as an adjective.
3. is fine. six-month functions as an adjective.
4. is incorrect for the same reason as 2.

[jack]

1. is incorrect. Try, six months (of) credit on electrical goods.

http://dictionary.cambridge.org/defi...8165&dict=CALD
1. The shop was offering six months' (interest-free) credit on electrical goods. (So Cambridge Dictionary is wrong? Or 'of' is omitted there because there is an aspotrophe after 'months'?)

[Casiopea]

1. The shop was offering six months' (interest-free) credit on electrical goods.

So Cambridge Dictionary is wrong? Or 'of' is omitted there because there is an aspotrophe after 'months'?)

A credit of six months ~ six months of credit
six months' worth of credit ~ credit of six months' worth

[jack]

six months' worth of credit ~ credit of six months' worth

Are these correct? What do they mean?
1. The shop was offering six months worth of credit. (If this is wrong, why?)
2. The shop was offering six months' worth of credit. (What's the point of using an apostrophe here?)

Thanks in advance.

[Casiopea]

Are these correct? What do they mean?
1. The shop was offering six months worth of credit. (If this is wrong, why?)
2. The shop was offering six months' worth of credit. (What's the point of using an apostrophe here?)

Thanks in advance.

six months (noun) worth (noun) => six months' worth

a worth of six months ~ six months' worth

[jack]

Thanks.

a worth of six months ~ six months' worth

1. six months' (Isn't this noun+noun too?)

[Casiopea]

It's adjective+possessive noun.

[Reemy]

http://dictionary.cambridge.org/defi...8165&dict=CALD

Are these correct? What do they mean?
1. The shop was offering six months' credit on electrical goods.
2. The shop was offering six months credit on electrical goods. (If this is wrong, why?)

3. The shop was offering a six-month credit on electrical goods. (If this is correct, why? Cambridge says 'credit' is uncountable? How come I can use the determiner here?)
4. The shop was offering a six-months credit on electrical goods. (Is 'months' supposed to be 'month'? Because it is an adjective?)

Dear Jack
Sentence #1 is correct, the –s genitive is used here with inanimate noun (temporal noun), as when we say "a week's holiday".

Sentence#2 has an incorrect form, the plural form is needless here. To express a period of time you can say six-month credit.

Sentence #3 is partially correct following the grammatical rule in expressing periods of time as in saying five- day visit. However it is incorrect to use a determiner (a) before "credit" which is uncountable noun.

As for sentence #4 as you said the construction six-month comes as an adjective (a premodifier) in which plural nouns usually become singular, so it should not be in plural form.