It is very easy to see that neo-Latin languages (Italian, French, Spanish, Portuguese and Romanian being the most well-known) are related because of their noticeable similarity. Some languages are so similar that their speakers can understand each other – they are mutually intelligible – like to some extent Portuguese and Spanish. What about other languages with less apparent similarity? How do we know that German and English are related or even English and Italian?
Just like in biology, relatedness cannot be based solely on similarity. Two languages are related if they can be traced back to a common ancestor. To do that, linguists examine sound changes that took place in words that existed early in the two lanugages. This is important because although the Italian dottore is very similar to doctor, this doesn’t prove that the two languages are related since it is a loan word that got into English in the Middle Ages. Sounds, however, change in a regular manner. If one change occurs, it usually applies in every situation providing reliable evidence that two words have the same origin, that they are cognates.
For example, In German the sound /b/ changed to /v/ in English:
Note how these words are very similar and they even have the same meaning. Now let’s look at how in Old English if a word starts with /d/, there is a similar word in Italian that starts with /f/:
do fare ’to do’
deer feroce ’wild’
dust fumo ‘smoke’ (originally both meant cloud)
door fuori ’outside’
In these cases the words are quite different even in meaning, but we shouldn’t be surprised since English is far more distantly related to Italian than German. Other distantly related languages are Finnish and Hungarian, the most widely spoken non-Indo-European languages in Europe that belong to the Finno-Ugric language family. All the other languages in Europe, apart from Finnish, Estonian, Turkish and Basque belong to the Indo-Eurpean language family. Using this comparative method, examining systematically repeating differences, linguists can rule out coincidences and state with utmost certainty if 2 languages are related. By doing so, they constructed the Indo-European language family of which you can find a link below. Enjoy!
Also check out this video by TED-Ed on the evolution of languages:
Sources and further reading (Hungarian):