I don't understand the formula for sodium hydrogen phosphate. Can you please tell me it and explain it?

Sodium hyroxide is NaOH.

Phosphoric acid is H3PO4.
So NaOH and H3PO4 react in different proportions to produce Na3PO4 (if all 3 H atoms are replaced)[leaving no H], Na2HPO4 (if only 2 H atoms are replaced)[leaving 1 H], or NaH2PO4 (if only 1 H atom is replaced)[leaving 2 H]. The PO4 ion is -3 and Na is +1. All you are doing is replacing a +1 H with +1 Na (or two or three as the case may be). I hope this helps but repost if it doesn't and explain in detail what you don't understand.

so how do i know which one is correct? If i understand correctly you said it could be Na2HPO4 or NaH2PO4...

also i don't understand how there can be +1 Na and then a -3 PO4...
wouldn't it then have to be Na3 to balance?

sorry i'm really lost

The names let one sort out which.

Name One
Sodium hydrogen phosphate
sodium monohydrogen phosphate
disodium hydrogen phosphate

Name two:
sodium dihydrogen phosphate
monosodium hydrogen phosphate
monosodium dihydrogen phosphate

I have seen all names used.

On your last question, remember than H has a charge of +1.

There is one sodium phosphate, Na3PO4, and 2 sodium hydrogen phosphates (one is sodium dihydrogen phosphate [NaH2PO4] which is also called monosodium dihydrogen phosphate and one is disodium hydrogen phosphate [Na2HPO4] which may also be called disodium monohydrogen phosphate.

Yes, IF all three hydrogen atoms are replaced on H3PO4, the acid, then Na3PO4 is what is formed because, as you correctly point out, PO4 is -3 and Na is +1. But when H3PO4, containing three replaceable H atoms reacts with a monoprotic base, such as NaOH, it CAN replace all three, or only 2, or only 1.
The charges, obviously, are PO4^-3, H2PO4^-1, and HPO4^-2. You can see that if we add 1 H ion to PO4^-3 we have left HPO4^-2 because -3 and +1 = -2 or if we add 2 H ions to PO4^-3, we have left H2PO4^- because -3 and +(2x+1) = -3 + 2= -1. I hope this is better.

The chemical formula for sodium hydrogen phosphate is Na2HPO4. Let me break it down and explain it!

In this formula, "Na" represents the element sodium. Sodium is a metallic element with the atomic number 11. Its symbol, Na, is derived from the Latin word "natrium." Sodium is highly reactive and it is commonly found as a salt compound.

"H" represents the element hydrogen. Hydrogen is the lightest and most abundant element in the universe. It is the atomic number 1 and is typically found as a diatomic molecule (H2). In compounds, hydrogen usually forms a bond with other elements.

"PO4" represents the phosphate ion. The phosphate ion contains one atom of phosphorus (P) and four atoms of oxygen (O). Phosphorus is a non-metallic element with the atomic number 15. It is widely used in various compounds, including fertilizers and detergents. The combination of phosphorus and oxygen forms a polyatomic ion called "phosphate."

The overall formula, Na2HPO4, indicates that the compound contains two sodium ions (Na+) for every one phosphate ion (HPO4²⁻). The charge balance is achieved by adding an extra hydrogen ion (H+).

So, sodium hydrogen phosphate is composed of sodium ions (Na+), hydrogen ions (H+), and phosphate ions (HPO4²⁻). The formula Na2HPO4 represents the specific ratio and arrangement of these ions in the compound.