What is the correct lewis dot structure for h2cf2?
is it linear or in box form?

Both H and F atoms are bonded to the central carbon atom with single bonds and it's in a box form.

Explanation: Carbon has four valence electrons, H has one valence electron and F has seven valence electrons. All elements try to complete their nearest noble gas like electron configuration.

Nearest noble gas to H is He and so it wants to complete two valence electrons. Rest of the elements try to complete eight electrons in their valence shell.

As H has one valence electron, it forms a single covalent bond to complete two electrons in its valence shell.

F has seven valence electrons and it needs one more to complete eight electrons in its valence shell and so it also makes a single covalent bond.

Carbon has four valence electrons and it needs four more electrons to complete eight electrons to its valence shell.

So, in Lewis dot structure of the carbon atom is the central atom and two H and two F atoms are bonded to it with single covalent bonds.

The molecular geometry of the molecule is tetrahedral(in box form).

Lewis structure of the molecule is attached.