1000 metres in 1 km
60 seconds in 1 minute and 60 minutes in 1 hour
Significant digits:
All non-zero digits are significant: 1234 has four sig. fig.
zeros before a digit are not significant: 0.00054 has two sig. fig. (5 and 4)
zeros between digits are significant: 4008 has 4 sig. fig.
zeros after a digit and behind a decimal point are significant: 7.40500 has six sig. fig. (so your teacher is correct 1.000 has four sig. fig.)
zeros afte a digit but not behind a decimal point are generally not significant (but this is not always the case, and this is why scientific notation is prefered) for example: 96000 is ambigous which digits are significant (I would say definitely two, ie 9 and 6) so using scientific notation you can show which are significant: 9.6 x 104 (2 sig. fig.) or 9.60 x 104 (3 sig fig) etc etc