Answer: The soil under your lawn can become compacted from foot traffic, and compacted soil excludes air (which roots need). You can fix compacted lawns by aerating every 2-3 years in either the spring or the fall. If you rent a power aerator, it will dislodge plugs of soil. Leave the plugs on the lawn (they'll decompose and break down in the rain). After aerating, spread a thin layer of sand or organic mulch over the top of the lawn and then water deeply.
Aerating doesn't eliminate weeds and clover, but will promote a lush, healthy lawn which should crowd out weeds. If your lawn is less than 3 years old I wouldn't aerate, but instead get rid of the weeds and then put the lawn on a regular maintenance schedule of mowing frequently, watering deeply (one inch per week) and feeding 4 times a year (April, June, September and early December).
Hope this answers your question!
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