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Relative pronouns connect a noun with identifying information: who for people, which for things, whose for possession. In a defining clause, that can often replace who/which. The pronoun can be omitted only when it is the object, not when it is the subject: the guide (who) we met; the guide who helped us.
In many words ending -ic, stress the syllable immediately before the suffix: do-MES-tic, eco-NO-mic. In many words ending -ious, the syllable before the suffix carries stress: de-LI-cious. Treat these as patterns with examples, not a rule without exceptions; check a dictionary for an unfamiliar word.