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A defining relative clause gives essential information identifying a person or thing: Students who practise regularly often gain confidence. Do not put commas around it. Who/that can refer to people and which/that to things. Only an object relative pronoun can be omitted.
Words ending -ion often have stress just before the suffix: communi-CA-tion, edu-CA-tion. With -ity, stress often falls on the syllable before -ity: a-BI-lity, oppor-TU-nity. Listen to the whole word and mark the stressed syllable rather than stressing the suffix itself.