All natural languages because of their nature as social phenomena are susceptible to various factors influencing their development. Therefore, apart from regional variations, social variations and personal fluctuations there are numerous differences between the form a language takes now and its past forms. The branch of science which investigates the present form and use of languages, as well as their past is called philology.
In the nineteenth century many philologists attempted to find some common roots of contemporary languages, as they noticed many similarities in their forms. Thus, certain family connections were traced and language genealogic trees were created. Up to now about thirty detailed trees have been made describing relations between 4000 distinct languages. The family tree of Indo-European languages consists of over twenty tongues.
The analysis of the relations between languages is possible because of common words that exist in those languages and derive from a mutual ancestor. Words that have a very similar form in different languages and share, or used to share their meaning are called cognates. For instance the English words mother and father are cognates of the German words Mutter and Vater. When two languages share a large number of cognates certain procedures are applied in order to reconstruct the original forms of the common ancestors.
The principles of such procedures are fairly simple: if in a set of cognates consisting of ten words seven of them begin with [k] sound and three with [g] sound according to the majority principle it is assumed that the majority of words have kept the original sound, while the rest changed with time. The most natural development principle describes the most common sound-change patterns which enable the reconstruction of sound changes in cognates:
* Vowels in the final position frequently disappear: vino -> vin * Voiceless sounds between vowels became voiced: muta -> muda * Stops turn into fricatives: ripa -> riva * If a consonant is at the end of a word it becomes voiceless: rizu -> ris
With the above mention set of rules it is possible not only to follow the history of language change, but also to determine the common ancestors of languages. However, there are many more sound changes which might occur in a long history of languages. For instance metathesis is a sound change which involves the change of position of two sounds within a word: frist -> first, hros -> horse, bridd -> bird.
One other type of sound change is called epenthesis and it is addition of a sound to the middle of a word: thun(o)r ->thunder, spinel -> spindle, timr -> timber.
One more type of frequently occurring sound change, however not to be found in English, is prothesis and it as addition of a sound to the beginning of a word.
Apart from the changes in the pronunciation of words there are some processes that influence the meaning of lexical items. Broadening, for example, is a process in which a words starts to be used with more general meaning than at the beginning of its use, as in the case of the English words: holy day meaning religious celebration, which broadened its meaning and turned into holiday which means a day free of work. Another process that influenced the meaning is narrowing which means that a word starts to be used in less general meaning than at the beginning, for instance: the Old English mete which meant any type of food and which is now spelt meat and denotes edible animal flesh.
Moreover, with time also the grammar of language might undergo some changes. In the case of English it is clearly visible. What seems to be one of the most significant changes in the English language is the loss of case endings. In comparison with its older versions English has now more fixed word order, but also strict rules regarding negation which forbid using multiple negations in one sentence, which was possible in the past.