Why do English speakers mix present and past tense?

A PhraseMix reader asked this interesting question:

I have realized that some people use present tense mixed with past tense while talking about past events. For instance, check out the following statement:

I wouldn't say the man lied to his surbordinates yesterday. However, it is clear that he exaggerated the improvements beyond what any leader has a right to.

I would have used "was" instead of "is" for the word in bold.

So the question is, why do English speakers mix present and past tense?

Thoughts can happen in the past or the present

When you're talking about a past event, you usually use the past tense to talk about things that happened:

He lied to his subordinates.

They left early.

When it comes to your thoughts and reactions, though, it's a little more complicated. That's because some thoughts happened in the past:

I knew he was lying.

I was glad that they left.

But you can also have thoughts in the present about the events that happened in the past. In that case, use the present tense to express your feeling and the past tense to talk about the action:

I think he was lying.

I'm glad that they left.

This explains the quote in the question above:

I wouldn't say the man lied to his surbordinates yesterday. However, it is clear that he exaggerated the improvements beyond what any leader has a right to.

The event happened yesterday ("the man lied", "he exaggerated the improvements"). However, "it is clear" is a thought that the speaker has right now. 

Reported speech and reported thoughts have their own time

When you're reporting what someone said or thought, there are two different times to think about:

• the time when the person said it or thought it
• the time that the person was speaking or thinking about

The first time is reflected in words like "___ said" or "___ believed", but the rest of the sentence depends on the second time. For example:

She told me that she wanted some ice water.

In this case, "told" is past tense because she said it in the past. "Wanted" is also in the past because she wanted the ice water at that time.

But sometimes people say things that are permanently true:

He taught me that knowledge is power.

But this is actually a point that native English speakers get confused about, so you may read or hear this as well:

He taught me that knowledge was power.

We sometimes switch to the present for dramatic effect

There are other reasons that English speakers switch tenses when talking about the past. For example, we often switch to present tense for dramatic effect when telling a story. For example:

It was last Monday. I had just gotten home from work, and I was in the apartment alone. I was watching TV. Suddenly I hear a strange noise coming from the bathroom. I get up and go to the bathroom door, which is closed.

When you tell a story this way, it helps the listener to imagine that they are in that situation.

English speakers make mistakes

Also, keep in mind that English speakers make mistakes. Sometimes when you're speaking quickly, you mix up tenses as your ideas change.

Can you think of any other examples of situations in which you've heard English speakers switch from past tense to present tense within a sentence?