I read somewhere that chronic renal failure can present with polyuria. then how is it that acute renal failure is the opposite?

Both can: Some types of acute renal insufficiency resolve with a large amount of urine production as the kidney regains some normal function. Some types of chronic renal insufficiency involve loss of protein or electrolytes and large amounts of urine production can result. Both acute and chronic renal insufficency have types where little or no urine is made.

Not exactly: Chronic renal failure in late stages may present with polyurea. Acute renal failure especially acute tubular necrosis may present with low urine output but need not be the case always. Acute renal failure during recovery phase may also be polyuric.

Polyuria: Chronic renal disease causes a defect in the kidney's ability to conserve water (urine) so you urinate more. Acute renal failure is usually more rapid severe damage that interferes with the kidney's ability to form water. If you can't make urine, you cannot survive more than a few days, so there is no chance to develop chronic renal failure.

Concentration loss: The kidney loses concentrating ability also, so that at a certain stage of renal failure polyluria can be the main complaint. Usually at night. (nocturia), when too much urine production interupts sleep.