ambos estão certos, na verdade em ingles o correto é dizer que quando voce sabe fazer algo, voce diga que "sabe como fazer", talvez tenha ficado confusa a minha explicação, mas olha exemplos:

Do you know how to do that?

I know how to dance

we know how to play soccer

mas voce pode dizer a mesma coisa trocando o "know how to" pelo "can"

Do you can do that?

I can dance

we can play soccer

(isso porque quando se usa os modal verb (nesse caso o can) não se usa infinitivo, então não coloca o "to"

O que ocorre eh que em inglês, usamos(to can) can ou can't + verbo para dizer que sabemos fazer ou não algo. Ex.: They can count ou They can't count. Se usarmos o verbo to know é necessário acrescentar esse how antes do verbo principal( o que indica o que sabemos fazer) Ex.: I know how to cook(Eu sei cozinhar) ou você pode usar to can e dizer: I can cook.

Algumas informações sobre isso de R. Murphy:

1. want/ ask help/ would/ Like /expect/ beg/ mean (= intend)/ would / - esse tipo de verbo é seguido de to, nesta estrutura: verbo + to: • We expected to be late. • Would you like to go now? • He doesn't want to know. Ou nesta: verbo + objeto + to: • We expected Dan to be late. • Would you like me to go now? • He doesn't want anybody to know. • Do you want me to come with you? (- não use that após want) Após o verbo HELP, o infinitivo pode ter ou não o to: • Can you help me to move this table? or Can you help me move this table?

2. tell/order/remind/warn/force/invite/encourage/ teach/persuade/get(= persuade – Esse tipo de verbo tem a estrutura verbo+ objeto+ to ... : • Can you remind me to call Sam tomorrow? • Who taught you to drive? • I didn't move the piano by myself. • I got somebody to help me. • Joe said the switch was dangerous and warned me not to touch it. • I was warned not to touch the switch. Com o verbo SUGGEST não se usa a estrutura verbo+objeto+to: • Jane suggested that I ask your advice. Com ADVIVE e ALLOW, duas estruturas são possíveis: verto+-ing sem objeto • I wouldn't advise staying in that hotel. • They don't allow parking in front of the building. Ou verbo + objeto + to: • I wouldn't advise anybody to stay in that hotel. • They don't allow people to park in front of the building.

3. Make and Let - Esses dois verbos têm a estrutura verbo + objeto sem to: • I made him promise that he wouldn't tell anybody what happened. • Hot weather makes me feel tired. • Her parents wouldn't let her go out alone. • Let me carry your bag for you.

um dos erros comuns do inglês é não pôr how após o verbo to know neste sentido. Você pode dizer: They can count. ou They know how to count. to Can significa to be able to do something = ser capaz de fazer alguma coisa – Gabriella can speak French fluently. You can swim, can’t you?

to know significa to have learned a lot about something or be skilful and experienced at doing something = ter aprendido muito sobre algo ou ser especializado e experiente em fazer algo - Gabriella knows how to speak French fluently. You know how to swim, don't you?

Não, o correto é "They know how to count."