While sipping a glass of beer, Joe recalled the day he had married Jill. Parallel thoughts should be expressed in parallel constructions. Boa constrictors have been observed to lie in the sun, to hang from limbs, and swallowing small animals. Boa constrictors have been observed to lie in the sun, to hang from limbs, and to swallow small animals. Avoid false predication.
Illogical constructions result.
The author's analysis assumes that the study population is isolated from other populations. The author's conclusion is based on her assumption that the study population is isolated from other populations. In most cases, the first verb in a sentence or paragraph establishes the tense of any later verb. Keep verbs within the same time frame. Avoid using split infinitives except when clarity or emphasis is clearly served by its use.