The role of etymology and grammar in language development and language relationship – regularities and rules
Morphological developments of Indo-European languages are known, but not formulated as laws: the loss of dual, of some cases and of the neuter, the change of root stems into the class of vocalic stems, the loss of the subjunctive, and the convergence of perfect and aorist, the replacement of the synthetic mediopassive by analytic categories.
The established criteria of the kinship between related language families or of the degree of kinship between two languages or within a group are morphological and lexical commonalities. Whereas lexemes are often borrowed, idioms are borrowed only between closely related languages. Grammatical categories can be created corresponding to patterns of neighbouring languages. Phonological borrowings are rare.

The role of etymology and grammar in language development and language relationship – regularities and rules
Morphological developments of Indo-European languages are known, but not formulated as laws: the loss of dual, of some cases and of the neuter, the change of root stems into the class of vocalic stems, the loss of the subjunctive, and the convergence of perfect and aorist, the replacement of the synthetic mediopassive by analytic categories.
The established criteria of the kinship between related language families or of the degree of kinship between two languages or within a group are morphological and lexical commonalities. Whereas lexemes are often borrowed, idioms are borrowed only between closely related languages. Grammatical categories can be created corresponding to patterns of neighbouring languages. Phonological borrowings are rare.