Some definitions:

INDO-EUROPEAN: (http://dictionary.reference.com/browse/indo-european)

A large, widespread family of languages, the surviving branches of which include Italic, Slavic, Baltic, Hellenic, Celtic, Germanic, and Indo-Iranian, spoken by about half the world's population: English, Spanish, German, Latin, Greek, Russian, Albanian, Lithuanian, Armenian, Persian, Hindi, and Hittite are all Indo-European languages. Compare family (def. 14).

 

Proto-Indo-European : ( http://dictionary.reference.com/search?q=Proto-Indo-European&db=luna)

The unattested prehistoric parent language of the Indo-European languages; Indo-European.

 

This video talks about the Indio-european languages, and how could languages such as English and Punjabi be related.”Sound correspondences can reveal how apparently unrelated languages are members of the same family”.

 

I’ve looked for information about this comparative method, and sound correspondence on the internet, and this is what I found:

Comparative method

(http://en.wikipedia.org/wiki/Sound_correspondence)

In linguistics, the comparative method is a technique for studying the development of languages. It requires the use of two or more languages. It is opposed to the method of internal reconstruction, which studies the internal development of a single language over time. Ordinarily both methods are used together. They constitute a powerful means to reconstruct prehistoric phases of languages, to fill in gaps in the historical record of a language, to study the development of phonological, morphological, and other linguistic systems, and to confirm or infirm hypothesized relationships between languages.

The comparative method was gradually developed during the course of the 19th century. The key contributions were made by the Danish scholars Rasmus Rask and Karl Verner and the German scholar Jacob Grimm. The first linguist to offer reconstructed forms from a proto-language was August Schleicher, in his Compendium der vergleichenden Grammatik der indogermanischen Sprachen, originally published in 1861.

Contrary to what is often assumed today, Schleicher and the other comparative linguists of the 19th century did not view the comparative method as a means to establish the validity of the language families they studied, which they considered to be already established through an interlocking web of lexical and morphological similarities. Characteristic is Schleicher’s explanation of why he took the then-radical step of offering reconstructed forms:

In the present work an attempt is made to set forth the inferred Indo-European original language side by side with its really existent derived languages. Besides the advantages offered by such a plan, in setting immediately before the eyes of the student the final results of the investigation in a more concrete form, and thereby rendering easier his insight into the nature of particular Indo-European languages, there is, I think, another of no less importance gained by it, namely that it shows the baselessness of the assumption that the non-Indian Indo-European languages were derived from Old-Indian (Sanskrit).

During the first half of the 20th century, the conviction gradually took hold that reconstructions arrived at through the comparative method were the only valid means to establish genetic (i.e. genealogical) relationship between languages. This has since remained the prevailing view among historical linguists. It is contested only by followers of Joseph Greenberg.

Demonstrating genetic relationship

In its application to the demonstration of genetic relationship, the comparative method aims to prove that two or more historically attested languages are descended from a single hypothethical proto-language by comparing lists of cognate terms. From these cognate lists, regular sound correspondences between the languages are established, and a sequence of regular sound changes can then be postulated which allows the proto-language to be reconstructed from its daughter languages. Relation is deemed certain only if a partial reconstruction of the common ancestor is feasible, and if regular sound correspondences can be established with chance similarities ruled out.

Developed in the 19th century through the study of the Indo-European languages, the comparative method remains the standard by which mainstream linguists judge whether two languages are related, with alternative lexicostatistical methods such as glottochronology generally considered to be unreliable as there are cases when lexicostatistics does not work at all. Potential problems with the comparative method have also arisen as a result of a number of advances in linguistic thought, in large part due to some of the "basic assumptions" of the comparative method. However, as Campbell (2004:146-7) observes, "What textbooks call the 'basic assumptions' of the comparative method might better be viewed as the consequences of how we reconstruct and of our views of sound change."