Learn Finnish: Tämä kukka tuoksuu raikkaalta.

This is because of the verb tuoksua (to smell). In Finnish, verbs of perception (like smelling, tasting, sounding, or looking) require the adjective to be in the ablative case, which means adding -lta or -ltä. Instead of using the basic form raikas, we must use raikkaalta.

What is the dictionary form of raikkaalta, and why does the spelling change so much?

The dictionary form is raikas (fresh). Words ending in -as or -äs undergo a stem change where the final s drops and the previous vowel doubles. The word also goes through consonant gradation, where the 'weak' single k strengthens to a 'strong' double kk. So, raikas turns into the stem raikkaa- before the -lta ending is added.

Could I use the verb haista instead of tuoksua in this sentence?

You wouldn't want to use haista here! Both verbs mean to smell, but tuoksua specifically means to smell good or pleasant, while haista means to stink or smell bad. Since smelling fresh is a positive thing, tuoksuu is the correct choice.

If I wanted to say 'These flowers smell fresh', would raikkaalta change into a plural form?