When can "as" be used instead of "when"?

Solution 1:

I believe "as" can describe a sense of simultaneous occurrence whereas "when" gives a sense of sequential occurrence.

When I reached the house I saw some guy walk out the back door. 1. You reach the house, action completed. 2. AFTER you reached the house, you saw some guy.

As I reached the house I saw some guy walk out the back door. 1. As you are completing the action of reaching the house you see some guy.

It can also be used to create distance in time (when) vs something in passing (as).

When I was younger, I would unload my friend's truck on the farm since I was stronger. (in the past)

I saw my friend unloading his truck on the farm, but as I was younger, I told him I'd do it since I was stronger. (in the present)

But just then, as he reached into his pocket, I saw the bale straps give way. (simultaneous events)

Example of "when" and "as" in the same scene: He raised the gun to her head to draw further attention to his apparent intent. With her body trembling, all she could think of was at this distance any attempt to escape would be futile. She knew that when he pulls that trigger it would already be too late. He tightens his grip to get ready for the kick but as he pulls the trigger the girl suddenly drops dead. A faint whirr and a glottal stop - the bullet piercing her spinal column - all from a trajectory that can be traced back to a rear window. "There he is!". He rushes to the window "Hey you there! Stop!". The assassin quickly disappears into the night, aided by an already fastened rope the dark figure dances over the adjoining apartment wall. Time to give chase. It's a long, long way down Spunkmeyer. Fortunately when things get this desperate, jumping out of an open window on the fifth floor doesn't seem so crazy. He looks back at his target. Her corpse lay limp on the floor. A growing red stain radiating from the exit wound.

Solution 2:

You can use as to mean when in conjunction with the past continuous. It is then synonymous with while. You have listed one example yourself. Here is another example:

• As I was loading the car, it started to rain.

I am not aware of any other tense in which this works. Even here it is somewhat problematic becuase it ccould mislead the listener into thinking you are using it with the meaning of because or since and will follow with a result:

• As I was loading the car, I had no time to help make the sandwiches.

Having just consulted Swan (Practical English Usage) in response to Naomi's question I need to edit my answer. Here is what Swan says:

A progressive tense is usually used for the longer 'background' action or situation. But 'as' can be used with a simple tense, especially with a verb like sit, lie or grow which refers to a continuous action or state: 'As I sat reading the paper, the door burst open.'

In this case, we would be able to substitute while for as but not when.

Solution 3:

"As" is used in temporal clauses as an alternative to "while", not "when". In your example "As I was going to St Ives, I met a man with seven wives" the sentence works with "while" instead of "as". Both of them are used to introduce an action in progress, as is the one in the Past Continuous form. On the other hand "when" introduces a temporal clause where the action either took place once or was repeated (the example you mention "Even when he had the chance to help, he did not" can be interpreted either way).