Learn English – “At” vs “by” in time-expressions

The past progressive tense is used to describe an on-going action that occurred at some time in the past, ie the action occurred over a period of time. The actual length of that time may or may not be specified. The action may or may not have been completed.

The past tense is used to describe an event that occurred at some time in the past. The duration of that event is irrelevant; the event is effectively treated as if it occurred at a moment in time. The event is always regarded as having been completed.

If you are talking about two actions in the past that are in progress simultaneously, then you must use the past progressive tense to describe both actions. In this case your original sentence would need to be used.

If you are talking about an event in the past that occurred simultaneously with an action that was in progress, then you should use the past tense to describe the event while using the past progressive tense to describe the action in progress. For example:

While he was taking a shower, his dogs ate his steaks.

or

His dogs ate his steaks while he was taking a shower.

If you are talking about two events that occurred simultaneously in the past, then you should use the past tense to describe both events. For example:

When he took his shower, his dogs ate his steaks.

or

His dogs ate his takes when he took a shower.

You should note that I changed 'while' to 'when' in my last two examples. Simultaneous clauses are often linked with the use of 'when' or 'while'. Other links such as 'and' or 'as' are also frequently used. Because 'while' indicates that a period of time has elapsed, it is frequently used at the beginning of a clause that uses the past progressive tense, and is rarely used at the beginning of a clause that uses the simple past tense. On the other hand, 'when' is rarely used at the beginning of a clause that uses the past progressive tense, and is frequently used at the beginning of a clause that uses the simple past tense.