It is quite regular and appears in other languages as well.
The reason may actually come from the Havlík's law. The -i in the plural is an actual -i that caused the yer between p and c to be weak and thus elided. In nominative the final yer was after the c and was elided instead and the yer between p and c was vocalized.
That's the general process for similar words, the word chlapec may have emerged after this process finished, but was created regularly, similar to other words ending -ec, gen. sg. -ce, nom. pl. -ci.
Compare свѧтьць (svętĭcĭ) -> světec / světci.
- this - "tento" (more formal), "tenhle" (less formal)
- that - "ten"
- the - often not needed, otherwise "ten"
- a - usually nothing
You just need to modify "ten" (and "tento", "tenhle") according to gender and number ("ta", "to", "ti" etc.).
And of course, on the English side, "those" and "these" are just plural versions of "that" and "this" respectively.