Learn German: Es handelt sich um einen Altbau; er ist teils verfallen, teils charmant. - It is about an old building; it is partly dilapidated, partly charming.

This is a two-part conjunction that translates to "partly ..., partly ...". Here, it connects two contrasting adjectives (verfallen and charmant) to show that the building has a mix of both qualities. You can use it to link adjectives, adverbs, nouns, or even whole clauses.

Why use the phrase Es handelt sich um instead of just Es ist?

Es handelt sich um is a very common, slightly formal phrase meaning "it is a matter of," "we are dealing with," or simply "it is." While you could just say Es ist ein Altbau, using Es handelt sich um sounds more descriptive and professional. Remember that this fixed phrase always requires the accusative case, which is why we see einen Altbau.

Why does the second clause start with er ist when a building is an "it" in English?

German pronouns must match the grammatical gender of the noun they replace. Because Altbau is a masculine noun (der Altbau), it must be referred to with the masculine pronoun er. Using es here would be grammatically incorrect.

Does teils change the word order if I use it to connect full clauses instead of just adjectives?