what is substance over form

For example an asset has life period of 20 years,I have given you it for lease for 19 years.So,out of 20 years useful life of asset You are only using it for 19 years which means you're only using it for major time period,So You have to record it as an asset(even though you're not legal owner) and you have to provide depreciation.Even though I'm legal owner I don't record it as asset.