What is the name of language that formed basis most modern languages?

There isn't one single language that formed the basis of most modern languages. It's more accurate to say that modern languages have evolved from a family tree of languages, with Proto-Indo-European being the common ancestor of many of the languages spoken in Europe, parts of Asia, and North and South America.

Here's why it's complex:

* Proto-Indo-European: This language is reconstructed by linguists based on similarities between languages like English, Spanish, Hindi, and Russian. However, Proto-Indo-European itself is not a language we can directly study.

* Language Families: Proto-Indo-European branched out into many daughter languages, which then further branched out. For example, Germanic languages (English, German, Dutch) are a branch of Indo-European, while Romance languages (Spanish, French, Italian) are another.

* Other Language Families: There are many other language families around the world, such as Afro-Asiatic, Sino-Tibetan, and Uralic. These families also have their own ancestor languages, and they are not related to Proto-Indo-European.

So, instead of one "basis" language, there are many ancestor languages that have contributed to the diversity of languages we have today.