I read in a grammar that "the soft character of a consonant is not indicated orthographically when it preceeds a soft consonant:
свет, летний = с'вет, лет'ний. However, if when the word varies the second consonant becomes hard, the first consonant being still soft, then ь appears:
возьми (take!), возьму".
I think that this example is little affortunate. According to what is said возьми should be возми, since з is soft because of being followed by a soft consonant.
I know that the origin of this ь is : в(о)зьм-ть ; "м" changes to "a" before consonant endings giving -> в(о)зьa-ть and finally взять. And newly recovered it all in возьму, ...
Since I see conflict between the rule given and the respect to the "making" of this and other verbs, perhaps the second part of what I say should be stronger than the first one, isn't it? Thanks.