The 是... 的 construction is used to draw particular attention to certain information in a sentence. It's often used to ask questions that seek specific information, or to explain a situation by emphasising a particular detail.

While not strictly tied to any "tense," the 是... 的 construction is frequently used when asking details about the past or telling details about the past.

Basic 是... 的 structure 
A 是... 的 sentence is formed with this structure:
subject+是+information to be emphasized+verb+的

This structure can be used to emphasise any detail, but most commonly it emphasizes time, manner, or place. Don't worry if this still seems a little confusing; lots of straightforward examples are coming up!

Simple Question Examples 
These are some of the most common questions asked using the 是... 的 construction and some possible answers:

You might be wondering, "can I still say the same thing without the 是 and the 的?" The answer is that in most cases, no, not really. While the 是 can sometimes be dropped, these examples sound weird without the 的. It's just a part of learning to ask questions naturally in Mandarin. You don't have to learn a "past tense," but you do have to learn this way of asking for details about the past, sooner or later.

When to use 是... 的 
As described above, the 是... 的 construction is used to draw attention to particular information in a sentence. wheras a sentence without a 是... 的 construction might describe a situation, a sentence with a 是... 的 construction explains it. 是... 的 is not generally used for reporting new information, but for adding important details that make the information clearer.

You could think of 是... 的 as being equivalent to saying one of the following in English:

"The situation is that"
"It's that ..."
"It was ... that ..."

If the sentence could be formed with one of those phrases in English, it can probably use a 是... 的 construction in Chinese. Usually the topic of a 是... 的 sentence has already been established.
Completed action

It's important to note that a 是... 的 construction also indicates that an action has been completed. However, this is not the purpose of a 是... 的 construction. The association with a completed action is more like a side effect of 是... 的. This means you shouldn't use 是... 的 just to indicate that an action is completed. Use the aspect particle 了 for that. Instead, use 是... 的 to draw attention to particular features of the action.

Negating 是... 的

是... 的 sentences can only be negated with 不, as 没 can not be used to negate 是. Place 不 before 是 as you would in a standard sentence.

Some examples: 

Note that negating a 是... 的 construction creates the implication that the action in the sentence was carried out, and only the detail emphasized by 是... 的 is being denied. So, in the last sentence, the implication is that 你 didn't go out with 她, but did go out with someone. So negative 是... 的 constructions would work nicely in the final scenes of detective dramas.

是... 的 and questions

是... 的 constructions can be made into questions in the usual three ways to form questions in Chinese:

With a question particle
With a question word
Within affirmative-negative questions

是 is often optional

Although this structure is called the 是... 的 construction, the 是 is nearly always optional. You will often hear this structure with 是 omitted, so be aware. The only time 是 is required in this construction is when it's being negated. Other than that, 是 is commonly omitted.

Some examples: 

wher to put 的 in a 是... 的 construction 
Up till now we've said that 的 appears at the end of the sentence in a 是... 的 construction. This is very often the case. However, if there's an object, 的 can appear before the object.

For example: 

Note that if the object is a person, 的 has to go at the end of the sentence. Otherwise it can go before the object or at the end of the sentence.

Sometimes this arrangement is useful for avoiding ambiguity in the sentence.