(a) It is foggy/there is a fog; It was very wet/there was a lot of rain; It won’t be very sunny/There won’t be much sun. (b) It is a long way to York; There is a long way still to go, (We have many miles still to go). It is time to go home, (We always start home at six and it is six now); There is time for us to go home and come back here again before the film starts, (That amount of time exists): (c) There is someone at the door. I think it’s the man to read the meters; There is a key here. Is it the key of the safe? (d) It is the grandmother who makes the decisions, (the grandmother, not any other member of the family); … and there is the grandmother, who lives in the granny-flat
[Los ejemplos de arriba pueden contribuir a evitar la confusión entre las dos formas: (a) ‘ «it is» + «adjetivo» ‘; ‘ «there is» + «nombre» ‘: «it is» va seguido de adjetivo y «there is» va seguido de un nombre; (b) tanto «it is» como «there is» pueden usarse aplicados a la distancia o al tiempo; (c) «it is» puede usarse para la identidad y «there is,» indicando, «presencia,» no; (d) «it is» puede usarse en «oraciones hendidas,» «cleft sentences» y «there is» («presencia«), no.]
A Practical English Grammar (A.J. Thomson & A.V. Martinet) 