You can use clues in the context of the equation to help you figure out what type of chemical reaction will be taking place. For example, in the case where hydrogen and oxygen bond to form water, this is a combination reaction. An example of a decomposition reaction problem might be zinc carbonate breaking into zinc oxide and carbon dioxide when exposed to heat. If you observe the formation of a solid that is insoluble in aqueous solutions, then that is a precipitation reaction; the solid product is known as the "precipitate." This is what happens in homes with "hard" water; a solid precipitate is formed by the solution and can clog pipes and faucets.
You can write a chemical equation by taking the elements involved and putting them into equation form. For example, if hydrogen and oxygen combine to make water, you immediately know that you are working with a combination reaction, and you would use words to write:
hydrogen + oxygen -> water
In order to balance this equation and write it out in skeletal form, you will need to write the actual molecules based on the symbols for each atom in the periodic table. One molecule of hydrogen is made up of two hydrogen atoms, one molecule of oxygen is made up of two oxygen atoms, and one molecule of water is made up of two hydrogen atoms and a single atom of oxygen. The above word equation becomes:
H(subscript)2 + O(subscript)2 -> H(subscript)2O
Notice that, in the example, H(subscript)2 + O(subscript)2 -> H(subscript)2O, there are two oxygen atoms on the left (reactant) side of the equation, but only one oxygen atom in your product on the right. Because atoms do not typically just disappear during a chemical reaction, you must balance the equation. In order to have a second oxygen atom on the product side of the equation, you would have to produce two molecules of water, so that the equation would become:
H(subscript)2 + O(subscript)2 ---> 2H(subscript)2O
At this stage you have two hydrogen atoms and two oxygen atoms on the reactant side, and four hydrogen atoms and two oxygen atoms on the product side. In order to balance the hydrogen atoms, simply add a "2" in front of the hydrogen molecule one the reactant side of the equation:
2H(subscript)2 + O(subscript)2 ---> 2H(subscript)2O
This balanced chemical equation tells us that two hydrogen molecules and one oxygen molecule can combine to produce two water molecules. This is a combination reaction.
A displacement reaction is one in which an atom reacts with a compound molecule to produce a different element and compound molecule. Similarly, a double displacement reaction is one in which two compound molecules react to produce two different compound molecules. A combustion reaction is one in which oxygen and another compound form water and carbon dioxide, releasing heat as a waste product. Knowing what type of reaction you are working with can help you determine how many products you will be left with after the reaction and, in some cases, what forms those products may take.