Affect v.s. effect, when would you use each of them?

More often than not, affect is used as a transitive verb (pertaining to a specified object), and effect is used as a noun.

COMMON APPLICATION OF EFFECT/AFFECT

The definition of effect usually used is in relation to a cause, a result of becoming operative, a degree of success, or an impression. For example:

The efficiency of the computational algorithm had a significant effect on the amount of time and memory required to perform the calculations.

Here, effect was a noun. The cause was the efficiency of the algorithm. The degree of success was significant, because it made an impression upon the observer's mind that the calculations took so much less time.

The definition of affect usually is something like, "to have an effect on something". For example:

The efficiency with which the computational algorithm was written greatly affected the amount of time and memory required to perform the calculations.

Here, affect was a transitive verb. The efficiency of how the algorithm was written, in effect, affected how quickly it finished.

THE CONFUSING SCENARIO

However, there comes an ambiguous case, since we have two very similar verb definitions for effect and affect according to Oxford English Dictionary:

In these cases, notice how "effect" tends to be more indirect as a verb, and "affect" tends to be more direct as a verb. For instance:

"This is beginning to affect how the law determines which of these relationships should be given legal recognition."

"This" directly affects "the [words of the] law", an object. This is stated outright.

"He even stated that he need not name every disease or body part, that God's power was effecting a multitude of cures all over the arena."

"God's power" effected, or brought about, "a multitude of cures" to many objects. Which objects? They were not specified, so the exact targets of the effect are open to interpretation.

"Affect" is typically used as a verb, meaning to influence or make a difference to something. Example: "The weather will affect our travel plans."

"Effect" is most often used as a noun, referring to a change that has occurred as a result of an action or other cause. Example: "The effect of the medication was immediate."