關於愛，我是個小學生。

別小看這兩個字，要分清楚得花些功夫呢：

先看字典怎說：

less·er adj. A comparative of little. 
1. Smaller in amount, value, or importance, especially in a comparison between two things: chose the lesser evil.
2. Of a smaller size than other, similar forms: the lesser anteater.

adv. A comparative of little. Less. Used before a participle: a lesser-known writer.

n. One that is lower in importance, rank, magnitude, or degree: the lesser of two evils.

lessadj. 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: no less a person than the ambassador.
3. Consisting of a smaller number. See Usage Note at few.

prep. With the deduction of; minus: Five less two is three.

adv. Comparative of little. To a smaller extent, degree, or frequency: less happy; less expensive.

n. 1. A smaller amount: She received less than she asked for.
2. Something not as important as something else: People have been punished for less.

再看看老外們怎說：

老外一：
My take on this rather tricky point of grammar is:

less can be an adjective (less money, less time)
less can be an adverb (a less pretty dress, it cost less)
less can be a pronoun (there was less than I expected)

lesser is always an adjective, and is preceded by an article (the lesser evil, a lesser person)
I would say that, of the two forms, lesser has generally less usage.

老外二：
less of the two - a smaller amount of both things under consideration: "I would prefer to see less of the two in the future."
lesser of the two - the thing that is smaller in quantity, amount, stature, etc. than the other: "I'll take the lesser of the two and you can have the other one."
One way to look at it, then, is that less refers to an amount of something that need not be compared to anything else, whereas lesser refers to the thing itself in comparison to something else (which need not be explicitly named).

老外三：
Less refers to quantity, lesser refers to quality. Your sentence could be rephrased as "substitute less punishment for more punishment." Here we are saying that the amount of punishment is smaller. But if you say "substitute the lesser punishment" you are saying the type of punishment is not as severe.
Forty lashes is less punishment than 100 lashes.
Pelted with rotten tomatoes is a lesser punishment than pelted with rocks.

最後是臉書 BCE 的看法：
Lesser常用於："To a lesser extent" (在程度減弱的情況下) ; "lesser of two evils" (兩害取其輕)。那麼，我們能否以less取代？不能。Less 是 "少"，Lesser 是 "小"。
以潑水為例，前者是不要5個水桶，1個就夠。後者是不要5L水桶，1L就夠。Extent是程度範圍，只可以「小」之，而不用「少」。

各位搞懂了嗎？

總之，"i will not in the future fetch a lesser position and earn less money for i studied less english everyday as i was a student only because english then was less interesting to me." 就對了~ XD