Lingua

Variable / veritable: one changes (varies), the other is a measure of truth or authenticity (veritas, Latin for "truth")

Statistics / statisticals: one is a collection of data, the other is an adjective that relates the accompanying nouns to statistics (in singular, the plurar doesn't exist)

Principal / principle: one is the main (principal) one, the other is an idea or concept (principle).

Verbiage / verbage: one is an overabundance of words (not a specific blurb of text), the other word doesn't exist.

Capital / capitol: one is the most important (capital), the other is the building used as seat of government (capitol).

A friend in China messaged me with this:

FEICHANG GAN XIE NI !!!!

Then / than : one is used as a connector, the other for comparisons.

Its / It's : one relates to possession, the other is a contraction of a noun and a verb and indicates action.

Their / They're / There : one relates to possession, the second is a contraction of a noun and a verb and indicates action, the third indicates a location.  The third can also be used as a modifier of other verbs; for example, "there is my hat", "there exists a god", "there will be blood".

There is no there there. So there.

Ya nimnoga gavaru pa Ruski.

Muito obrigado. Eu no falho portugues.
Mais eu falho espanhol..