Why don't priceless and worthless have the same meaning?
And why are flammable and inflammable synonyms rather than antonyms? (OK, I know inflammable is from the verb inflame, but superficially it seems wrong.)
The reason why is tied up with the way our language is derived from a variety of roots, Ruth. It gives us more opportunities to invest theoretically similar words with very precisely differentiated shades of meaning:
childish, childlike, infantile, puerile are pretty much synonyms, but each conveys a different attitude. The first two, being derived from Germanic tongues being more down to earth, less judgemental than the second pair, derived from Romance languages.
Why English is so hardWe'll begin with a box and the plural is boxes;
But the plural of ox is oxen not oxes.
Then one fowl is goose, but two are called geese;
Yet the plural of moose should never be meese.
You may find a lone mouse, or a whole lot of mice,
But the plural of house is houses not hice.
If the plural of man is always called men,
Why shouldn't the plural of pan be called pen?
The cow in the plural may be cows or kine,
But the plural of vow is vows, not vine.
And I speak of a foot, and you show me your feet,
But I give you a boot - would a pair be called beet?
If one is a tooth and a whole set are teeth,
Why shouldn't the plural of booth be called beeth?
If the singular is this and the plural is these,
Should the plural of kiss be nicknamed kese?
Then one may be that and three may be those,
Yet the plural of hat would never be hose.
We speak of a brother, and also of brethren
But though we say mother, we never say methren.
The masculine pronouns are he, his, and him
But imagine the feminine, she, shis and shim!
So our English, I think, you will all agree,
Is the trickiest language you ever did see!