Cerno

In Latin, “cernō” means “I separate”, “I distinguish”, or “I decide”. Especially, its derivative “sēcernō” means “I separate” (with the prefix “sē-“ implying the meaning of “apart”, which can be seen in the word “separate” itself). Also, “dēcernō” specifically means “I decide”. Cernō and its derivatives explain lots of basic English words.

discern, secern, secret

These words more or less have the sense of “to separate”. “Discern” baically means “to perceive or distinguish”, while “secern” means “to separate”, although the latter is rare today. “Secret” is a bit different from the other two, but can be considered as “separated from public knowledge”.

excrete, secrete

These two words are pretty similar in meaning and form. With the prefix “ex-“ (meaning “outside”), “excrete” makes sense because it specifically refers to the act of discharging waste outside of the body. On the other hand, “secrete” is not really about “outside”, it’s just about producing (separating) a certain liquid.

certain, decree

These words are more related to the meaning of “to decide”. The perfect passive participle of cernō is “certus”, which is exactly where “certain” (decided) comes from. For the record, “decree” means “a statement of an official decision”.

The word “decide” looks similar, but it seems to have a different origin.