Should be, should + V , MUST+V

其實你嘅理解大致都啱。

(1) should 同 must + verb (root form) = modal verb
eg. You should go to bed early.
You must not smoke in a non-smoking area.

(2) 如果冇verb，就用 be (auxiliary verb)，通常再加 adjective。
eg. You should be proud of yourself. (proud is an adjective).
eg. You must be kidding. (kidding is a gerund serving as an adjective)

你例子中 should be less 嘅 [less] 唔係 verb， 係 adjective。所以都符合以上規則。

不過，重有：
(3) should/must + be + verb (past participle form) = modal verb in the passive voice.
eg. This secret must be kept.
eg. This secret should be kept.




2010-05-11 10:02:28 補充：
Thefreedictionary:
less (ls)
adj. A comparative of little.
1. Not as great in amount or quantity: had less time to spend with the family.
2. Lower in importance, esteem, or rank: .......
3. .....
adv. A comparative of little.
To a smaller extent, degree, or frequency: less happy; less expensive.

less = 副詞
should/must後面+be+副詞/形容詞/動詞+ing/名詞都得,睇意思

動詞係指時態, he should be eating now=佢e+應該食緊嘢
she should be fat to eat so much=佢食咁多嘢,應該肥嘅
you should be a girl=你應該係個女仔
(意思可能有少少怪,但都係講文法啫)

must比should更肯定,不過用法一樣,must重有必須嘅意思
you must be a liar 你實係大話精.

如果要跟 adj 就一定要有be
e.g. It should be fine. or It must be fun.

如果後面跟個v就唔駛要 be
e.g. It should work. or You must do your homework.

其實 should/must + 個be
be的意思就是一個verb~ 一句句子要有verb~ 而呢個be就係咁用到喇