1.The house is so large that they can't paint it in a week 
=> if the house...was so large,they could paint it in a week......
2. Tam does not feel happy because he can't go with us 
=> If Tam...felt happy,he could go with us........
3. The rain is very heavy. That's why we can't come on time 
=> if the rain.was very heavy,we could come on time..