What does Historical Linguistics study?

Off

Historical Linguistics explores different aspects of language change. The most commonly studied areas in historical linguistics are:

Etymology: Studying the reconstruction and origin of words.
Analysis and description of multiple speech communities.
Tracing (as far as possible) the history of language. This includes Sanskrit, Latin, Old English, and also modern languages, such as German, Italian and Japanese. This process also involves grouping languages into categories, or “families”, according to the extent to which those languages are similar to each other.
Describing and analysing changes of any type which have occurred cross-linguistically and within a language itself. Languages can change in any area of language; phonology, syntax, morphology and orthography are only a few of the areas which could be considered.
The construction of a framework of theories which can account for how and why languages change.

Sub-fields

As is evident, the study of Historical Linguistics concerns many different topics. For this reason, it is comprised of different sub-fields.

Comparative Philology (or Comparative Linguistics) concerns the comparison of cross-linguistic features in order to establish the relatedness of languages.
Etymology concerns the study of word histories. This process may involve answering the following questions:

Why did a particular word enter the language in question?

Where does that word/root come from?

How has its orthographic, phonological and semantic value changed over time?

Dialectology concerns the historical study of dialects. Features which are usually a matter for debate are grammatical variations between two dialects, and phonological changes within a dialect over time.
Phonology concerns the study of the sound systems which exist (or have existed) in a specific language. Studies in phonology can also concern comparative approaches to language studies between different time periods, dialects and languages.
Morphology/Syntax concerns how the means of expression of a language evolve over time. It is usually focused on inflectional systems, grammatical structures, and word order.