Negative comparative

Question

For making comparative forms of adjectives, we add “more” before adjectives with more than one syllable and other than those two-syllable adjectives ending in “y”. So for example:

“This watch is more expensive than yours.”

And for other adjectives we add “-er” to the end of the adjective. So:

“He is healthier than his wife.”

In both above examples, the comparatives help to convey that the subject is at the higher level about the mentioned adjective: “The price of this watch is higher than yours.”

How we can convey the meaning that the subject is at the lower level than another thing? For example:

“Regarding his health, he is worse than his wife’s condition.”

If I want to rearrange the above sentence, is the following sentence grammatically correct?

“He is less healthy than his wife.”

And if the above sentence is grammatically correct, should we use “less” for all kinds of adjectives? I mean one syllable or more than one syllable? For example is the following sentence grammatically correct?

“It is less cheap than that one.”

Answer 1

There are some comparatives that are almost always made with a suffix ("bigger"), some that are always made with "more" ("more intelligent"), and some that sound fine either way.

In the case of "less", there is no suffixed alternative. No matter how short the adjective, the opposite of "more X" is "less X". So "less red", "less wet", "less mad", "less tall", etc, and similarly "least red", "least wet", "least mad", "least tall".

Where a word has a well known opposite, we usually tend to use the ordinary comparative ("-er" or "more") rather than using "less". For example, it's relatively uncommon to say "less good" (we tend to say "worse") or "less bad" (we tend to say "better"), and it's uncommon to say "less tall" or "less short" (we tend to say "shorter" and "taller" respectively). But there's nothing grammatically incorrect about "less good" or "less big" or "less stupid" or "least easy". They are just less usual choices, and in some cases they might seem humorous.

Answer 2

For your example, you can certainly say
"He is less healthy than his wife."
There are other ways to express that, too:
His wife is healthier than he is.
He is not as healthy as his wife.

The last example is the negation of the expression
He is as healthy as his wife.
That is an equative expression. It could possibly mean that the states of their health are equal, whether good or bad. But, in common use, it has a positive sense: that his wife is healthy, and he is equally healthy.

The negation of the equative expression, "not as healthy as", could possibly mean that their states of health are unequal, no matter which is better, but in common use it means that the first mentioned is less healthy than the second. So, "A is not as healthy as B" means A's health is worse than B's.

For your second example, a more likely expression is "It is not as cheap as that one."
The logical meaning is that they are not equally cheap, but it will be understood to mean "it" is more expensive than "that".

And there are other ways to say it:
That one is cheaper than this one.
This one is less cheap than that one.

There's a discussion of comparison involving adjectives here that gives a wider view:
Cambridge Dictionary comparative adjectives

Answer 3

Most suffixed comparative and superlative forms are optional; “more healthy” is just as correct as “healthier”. There are only a few irregular words that require the suffixed forms —or change completely, like good/better/best.

There is no suffix for “less”; you can only say “less healthy”. Notably, this applies even to words that don’t allow the “more” form, so “less good” works even though “more good” doesn’t.