Mnemonic rules, indeed, help quickly remember some ideas that can't be explained, but only memorized.
English has many words which, due to historic or other reasons, while having different meaning, are written similarly to each other. For an English learner, it becomes easy to misspell a word, e.g.:
I want to loose weight is incorrect.
I want to lose weight is correct.
How mnemonic works
Say, you are trying to convey some meaning. Let it be "losing weight", or "getting slim".
There are two words with very similar orthography:
- "loose" - to make less tight, or to unfasten
- "lose" - to cause something to stop being in one's possession (correct)
The problem is that you don't see a logical link between meaning and spelling.
However, if someone tells you:
Lose has lost one of its O's
it will clearly tell you: the word stopped having one of its letters, or the word lost it. So, now you have:
I want to say about something lost.
The word "lose" has also lost one of its O's
Hence, I must use the word "lose".
This way, mnemonics is a tool for artificially linking meaning and spelling.
Usually, it is sufficient to learn a mnemonic rule for a single word of a pair, and by recalling one rule, you will know that the other word will obviously have another spelling.
An extra benefit of mnemonic rules is that they usually have humorous sense. Indeed, you can imagine a poor word that walked somewhere and lost one of its letters. The good thing is that jokes are better remembered by human's memory. This makes mnemonics a powerful instrument, especially if you are a beginner and can't (yet) remember formal grammar rules.