<em>Tension,</em> <em>stress,</em> and <em>pressure</em> are nouns. <em>Tense</em> is an adjective meaning <em>under tension.</em>

First, I will speak about physical objects. <em>Tension</em> means pulling or stretching on something long and thin, like a rope, or a wire, or a cable.

<em>Two teams were pulling on a rope playing tug-of-war. They put tension on the rope.</em>

<em>This monofilament fishing line is rated 90 kg test. If you put it under more than 90 kg of tension, it will break.</em>

<em>Pressure</em> means applying a pushing force over an area, possibly tending to crush something or to inflate something. In engineering, it is measured in <em>pascals</em> or <em>psi (pounds per square inch).</em>

<em>The sticker on my car door says that the tires should be inflated to a pressure of 200 kPa.</em>

<em>Submarines that dive too deep can be crushed by the pressure of the water.</em>

<em>Reinforced concrete is very strong because the concrete resists compression and the steel reinforcing bars resist tension.</em>

<em>Stress</em> refers to a bending or twisting force.

<em>"My feet just kept hurting more and more. The doctor said the impact from running had put the bones in my foot under stress, and had caused tiny stress fractures."</em>

When speaking of human emotions, <em>stress</em> and <em>pressure</em> are similar. <em>Pressure</em> suggests time.

<em>"I'm working under a lot of pressure right now, we have to finish the project by the end of the month."</em>

<em>Stress</em> means more generally that you cannot relax or feel good, something is always irritating you.

<em>"The pandemic is causing all kinds of stress. There is financial stress because I'm not working. The kids are at home and they are driving me crazy. And I worry about getting stick myself."</em>

<em>Tension</em> often refers to a difficult personal situation between two people who want two different things. They are figuratively "pulling in different directions."

<em>"Jim and Joan did not look happy. You could see the tension between them. They had come to a difficult point in their marriage. Jim wanted to stay in Chicago, Joan wanted to move to Spokane."</em>

I really like Dan's explanation. Another way of explaining pressure is If somebody or something has certain expectations from you, and it causes you to feel worried or stressed, you can say that you are "under pressure".

ex: My parents are putting a lot of pressure on me to do well in school.