What should be used and why not other one? Fit ou fut

Bonjour Ankit,

‘Fut’ is Le Passé Simple of ‘être’ whilst ‘fit’ is Le Passé Simple of ‘faire’.

In this particular sentence, ‘ to be careful to do [sthg]’ is translated with the French expression ‘faire attention à + [sthg]’. Therefore you are looking at conjugating the verb ‘faire’ in Le Passé Simple, as per the topic of this lesson.

See following link: recognise-etre-in-le-passe-simple

I hope this is helfpul.

Bonne journée!