Probably the most useful tool in a language learner's inventory is: his or her own language.  Although the amount of help that language can be depends on what language and what language he or she is trying to learn.  The things that could be similar could range from morphology, idioms, syntax, phonetics, and phonology.  But when we have two words in two different languages which are similar in one of three ways and mean the same thing, you get a cognate. For purposes of practicality I will have to use English examples to other languages.

Cognates exist for a lot of reasons, one being that languages which have a common ancestor will probably still have similar vocabulary, morphology, etc.  Another reason, especially in modern days, is the introduction of more "universal" words.  This is why the words Telephone and Automobile are usually phonetically similar is most languages

Cognates are wonderful, they make learning a language a piece of cake.  Why wouldn't it be easy to remember a word if it's about the same as a word in your own language?  But there are different kinds of cognates.  I have split cognates into 3 different categories; 1) phonetic 2) orthographic and 3) both.

The first, phonetic is a cognate that sounds similar in both languages, but because of spelling reasons don't look similar.  One of my favorite examples is that of Spanish <que> to Italian <che> which are both pronounced /ke/.  This is similar to German, Norwegian, Swedish, and Danish <ja> to English <yeah> which have similar phonetics but are different in pronunciation.  Another simple example in German is <Kabel> which means <cable>.  I would say that these are bit on the rare side, when it comes to the total when it comes to cognates.  The word <jacket> in Spanish is <chaqueta> which is pronounced almost the same (the difference is voicing and a suffix).  It also is most common when it comes to languages which don't have the same lettering systems as each other.  <πορτα> is the same as Italian <porta> in both meaning and pronunciation. 

The second we have is Orthographic.  Orthographic means that the word looks the same in both languages.  This has many varying degrees, most of the time it involves some affix (a suffix, prefix, or infix) such as a different ending or such.  Sometimes it's the same, either way it is easily recognized by a non native speaker of that language.  These are words like <rail> spelled the same in Both Spanish and English, but the Spanish would naturally be different since it has a different sound with the <ai> /aj/ unlike the English /æ/ sound. The word <original> which also spelled the same in both languages but because of the difference in pronunciation of the <gi> in Spanish it ends up being pronounced differently.  The word <chance> in French and English don't really sound alike and they mean very similar things.

The last kind is the best, because it is the easiest to recognize.  That is because it both sounds and looks like the word.  Sometimes, mind it sounds like there is a different accent or slightly different vowel, but on the most part it is easiest to recognize.  The word <hand> in German and English, although pronounced slightly different is most likely recognizable in both spoken and written language.  The word <era> or <color> which is spelled the same in both Spanish and English could easily be recognized in both.

Now there is a bit of warning I must give when it comes to cognates.  It's good to help your remember when a word is a cognate, but I wouldn't rely on them.  There are plenty of False cognates.  <Chef> in German doesn't mean chef, but it means boss instead.  Many languages have false cognates or words we think are similar, but don't always assume it.

Timothy