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The word swam is the past simple tense of the verb to swim. To swim means

1. to move in water by movements of the limbs, fins, tail, etc.
2. to float on the surface of water or some other liquid
3. to move, rest, or be suspended in air as if swimming in water
4. to move, glide, or go smoothly over a surface

Some example sentences:

• He swam past lazily.
• We swam in the pond yesterday.
• Today we swam in the river.

If you need or want to review this concept, here is A Review of Active Tenses in English