Final answer:
The sentence that is not in the past tense is 'I fall every time I go skiing.'
Step-by-step explanation:
The sentence that is not in the past tense is: I fall every time I go skiing. In this sentence, 'fall' is in the present tense. The other sentences ('They fell in love at first sight,' 'The leaves were falling through the crisp autumn air,' and 'Little Timmy has fallen into the well!') are all in the past tense.
Learn more about tense in English sentences