My grammar weakness is that I’m forever mixing up the correct use of which and that, and less and fewer. This is probably just because my brain has been too lazy to consciously register the key differences.
I hope by redefining the rules here, I’ll finally know whether to get excited or begin an internal ranting whenever I see the “12 items or less” sign at the express supermarket check-out aisle.*
that vs which
Both that and which refer to objects.
The key difference is that adds essential information to the sentence while which only adds incidental information, or a sub-clause.
As a rule of thumb which usually requires a comma, and the following sub-clause could be removed and the sentence would still make sense. For example:
I want to buy the mini in Kingsland, which I saw advertised on trademe.
In this case the subclause following which only adds supplementary information about the mini, it is not essential information. I could remove everything from the comma onwards and the sentence would still make sense.
However, that provides essential information, and forms an integral part of the sentence.
If I now instead say:
I want to buy the mini in Kingsland that I saw advertised on trademe.
The sentence takes on a different meaning, implying that of all the minis I saw in Kingsland, I want to purchase the one I saw advertised on trademe.
less vs fewer
Fewer refers to a specific countable number of items, while less refers to a vague quantity. (Unless talking about money, then less is typically used, e.g. I want to pay $1200 less for the mini.)
So, I have looked at fewer ladas than minis, as the number of ladas and minis I have viewed are countable (although depending on their condition and disassembly that could be disputed).
But, I have seen less mini traffic, because (theoretically) you cannot have a single unit of mini traffic.
What this means is that I want to pay less for the mini on trademe, which is yellow, but the mini that is on Copeland Street has had fewer owners and might be better.
*And indeed it should read “12 items or fewer”.