1. Can any number be written in standard form?

Yes, any number however small or large can be written in standard form.

2. How does a calculator show standard form?

It varies from calculator to calculator but many show 3.45 × 102 as 3.45 02

3. What does the ENG button on a calculator do?

The ENG button converts between different metric units.

e.g. To change 5634 metres into kilometres press:       5.634 03

 i.e. 5634 = 5.634 km

4. Is it necessary to show the (×100) part, when writing a number between 1 and 10 in standard form?

Yes, to be in standard form the number must include the ×100 part.