Learning a language is based on studies of structure (grammar) and meaning (semantics). The former includes the composition of words, their syntax, and sounds. The latter includes denotations and connotations. All the material that follows is based on second language acquisition, defined as a language consciously acquired or used by its speaker after neurological puberty (this is closely linked to Critical Period Hypothesis).
Learning a second language can only be realistically achieved with stages . Whereas the acquisition of a first language is a natural process, learning a second language is a conscious one .
Unless one is only intending to learn the written form, first is sound recognition and expression. This is one section, along with basic numbers, that should be rote-learned.
See also the presentation The Worst of English.
In late 2015 I started using Duolingo and have since completed the following skill trees:
- Esperanto (English to Esperanto) (March 2016)
- French (English to French) (May 2016)
- German (English to German) (July 2016)
- Spanish (English to Spanish) (November 2016)
- anglais (English from French) (March 2017)
- Italian (English to Italian) (March 2018)
- Englische (German to English) (June 2018)
- allemand (French to German) (November 2018)
- Portuguese (English to Portuguese) (December 2018)
- Französisch (German to French) (December 2018)
- Inglese (Italian to English) (December 2018)
- Esperanto (Spanish to Esperanto) (December 2018)
- Russian (English to Russian) (December 2019)
- Latin (English to Latin) (December 2019)
- Indonesian (English to Indonesian) (June 2020)
- Chinese (Mandarin) (English to Chinese) (June 2021)
I have since had reason to raise a few questions on the 2022 update of the Duolingo user interface.
 Rod Ellis, The Study of Second Language Acquisition, Oxford University Press, 1994
 Stephen D Krashen, Second Language Acquisition and Second Language Learning, Prentice-Hall, 1981