A general truth is a fact about the world that is always true — not just now, not just yesterday, but at all times. It applies to scientific laws, natural phenomena, geographical facts, biological processes, and any statement that describes how the world consistently works.
Think of a general truth as something you could write in a textbook. It was true before you were born, it is true now, and it will be true after you are gone. The Present Simple is the tense for this because it describes permanent, timeless reality — not a specific moment in time.
Imagine time as a line stretching from the far left (the distant past) to the far right (the distant future). A general truth sits above that line — it belongs to all points in time at once. It is not a specific event that happened and finished (that would be the Past Simple). It is not something happening at this exact moment (that would be the Present Continuous). It is simply how things are.
The grammar is identical to Present Simple for habits. The only difference is what you are talking about — the world, not your personal routine.
✗ The Earth is orbiting the Sun. · Water is boiling at 100°C.
✓ The Earth orbits the Sun. · Water boils at 100°C.
The Present Continuous means something is happening right now, at this moment. A general truth is not happening — it simply exists. When you say "The Earth is orbiting the Sun", it sounds as if you are watching it happen in real time. For timeless facts, always use the Present Simple.
✗ Water boiled at 100°C. · Light traveled faster than sound.
✓ Water boils at 100°C. · Light travels faster than sound.
The Past Simple suggests the fact was true in the past but may no longer be true now. Unless a fact has changed (e.g. a law was changed, a species became extinct), always use the Present Simple for truths that continue to be valid.
✗ The Sun rise in the east. · A magnet attract metals.
✓ The Sun rises in the east. · A magnet attracts metals.
Even when stating a fact, the third-person -s/-es rule applies. Singular subjects (the Sun, a magnet, water, light, iron) all require the third-person form of the verb. This is one of the most frequent errors with general truths in English.