1. In the first group (healthy, canteen, crossword, worker), the word with a differently placed main stressed syllable is **canteen**. The stress pattern for the others is on the first syllable, while "canteen" has its stress on the second syllable.

2. In the second group (apartment, gymnastic, volleyball, karate), the word with a differently placed main stressed syllable is **gymnastic**. The stress on "gymnastic" is on the second syllable, while the others (apartment, volleyball, karate) have their primary stress on the second syllable as well, but "gymnastic" is a three-syllable word where the stress is atypically placed compared to the others' primarily two-syllable stress patterns.