The subjunctive mood is used in Spanish after certain expressions. These include:
1) Expressions of uncertainty e.g - Dudo que...
2) Expressions of feeling e.g - Siento que...
3) Expressions of wish/intent/command e.g - Quiero que...
The formation of the subjunctive in Spanish is very easy. There are 3 simple steps.
1) Take the yo form of the present indicative. e.g - Como
2) Drop the -o ending.
3) Add the subjunctive endings which depend on whether the verb ends in -ar or -er/-ir.
-e, -es, -e, -emos, -éis, -en
-a, -as, -a, -amos, -áis, -an
As with most tenses, irregularities exist in the subjunctive. If the verb is irregular in the present indicative, you use this irregular stem to form the present subjunctive.
E.g - tener -> tengo -> teng-
conocer -> concozco -> conozc-
There are also 6 verbs which are fully irregular in the subjunctive, these need to be learnt:
Dar, Estar, Haber, Ir, Saber & Ser
I want you to go to the supermarket. - Quiero que vayas al supermercado.
We doubt that they will eat it. - Dudamos que lo coman.
I don't believe that he has a new car. - No creo que tenga un coche nuevo.
There is no such thing as the future subjunctive in Spanish, the present subjunctive is used to express this. This is why 'coman' is used in example 2.