What skills can be improved by learning by heart poems in the target language? Won't this inhibit the learning process as poems sometimes disobey common grammar rules?
TL;DR Learning poetry in a foreign language can have a net beneficial effect in areas of vocabulary, language comprehension, and grammar (depending on the learner).
Poetry in general is constructive
Poetry typically uses illustrations and/or figurative language. Immersing yourself in such things can do you so much good. Not only can it strengthen your language comprehension skills, but also your vocabulary (idioms), imagination, and creativity, and potentially your sense of rhythm. Especially if the poem is memorized, pronunciation skills can improve as well.
In your target language, the complex sentence structure and vibrant language of poetry are even more beneficial to the language learning process. The use of polysemous words also enforces in learners a deeper understanding of vocabulary and grammar. source
It uses unconventional grammar
Yep. Poetry is probably going to break the rules of grammar. However, assuming that poetry is not your only resource for language study, I believe that the nonconforming grammar can be easily enough understood, and once that occurs, the learner's grammar can improve even more.
Otherwise, even if the learner picks up some less-than-ideal grammar from poetry, the good still outweighs the bad. A teacher, native speaker, or even a little research should be more than enough to correct any errors.
In conclusion, if you can, definitely study poetry in your target language. You won't regret it.
(Note: I think this is a somewhat subjective question, because everybody learns differently, so I've attempted to write a somewhat broad answer. However, I do think this is still a question fit for this site.)
Learning poetry is important to the cultural aspect of a language. Depending on your outcomes and goals for learning a language, poetry may or may not help you as much as you would like.
First, (almost) any exposure to a language will be beneficial as your subconscious begins constructing rules and observing patterns regarding that language. One study observed that a group of random people who were exposed to Korean, Japanese, and Chinese for 30 minutes (both written and spoken) were able to distinguish between the languages when presented with them one week later.
Second, many idioms and expressions come from poems or folk stories of that region and communicate a lot in a few words. For example, in English, we may say "Well, it's better to reign in Hell." and many people will know the subsequent phrase from Paradise Lost of "Than to serve in Heaven." Another example in Chinese, There is a famous poem that talks about procrastination, but most people will only quote the first one or two lines from the poem (明日复明日，明日何其多).
Overall, poetry will help you in your communication of a language because it is closely related the cultural experience of the people and they will connect to it and see you as caring about their culture as well as their language.
Comparing to the other methods of studying, learning poems has it own pro's and con's.
- Arguably, remembering a rhymed verse may be naturally easier than remembering a plain text of an equivalent size. It is very individual for every learner, however.
- Verses naturally have a certain meter. Learning poetry is extremely helpful for a learner to grasp the rhythmic aspect of the language. This becomes especially useful if one is learning with no aid from the native speakers.
- Learning poetry is fun, unlike "these boring lessons". If you teach kids, they will arguable enjoy it more than plain lessons.
- By knowing poetry of the language you're studying, it seems to be helpful to get treated with respect by the native speakers. This, in turn, may lead towards getting more help from them to a language learner.
Personally, I've remembered the full name of Bangkok city by remembering this song (Youtube) and Thai National Anthem, even without exact understanding of each individual word. I found the locals treated me with more respect because they see me expending some effort toward learning.
- Very often, writers of poetry step off the standard grammatical constructs, in order to achieve some artistic effect. Remembering those, before understanding the proper, grammatical ways to construct a phrase, may harm one's study.
In languages with lexical tone, the song lyrics may follow the music (ignoring the proper tones). Compare these two songs:
- Poetry may use obsolete words or constructs.
So, in order to mitigate the disadvantages of language learning by reading poetry:
- ensure you learn from modern poetry, at least, at the beginning stages;
- work with different sources, authors, and books.
- This question at Linguistics.SE may provide with some more details;
Poetry has been proven that it can help students improve their fluency by repeated reading, which also helps with improvement of fluency. Performance repeated reading (like repeated reading but with a performance) has also proven very, very effective. Though grammar might be a little off, poetry has its place in teaching students.
Articles have also shown the effects of poetry on the fluency of students, like this article. From personal experience, poems are more fun to read and bring out more emotion and is more thought provoking, showing a true understanding of the text thus better fluency.
The advantage of learning poems is that you learn the way the language is actually used, and by unusually sophisticated people who write these poems. Not everyone can write a poem, even in their native language.
There is a "downside" in that they sometimes disobey grammar rules. But they don't do this "randomly." There is usually a "method to their madness," and if you learn the whys and wherefores, you will get a deeper insight into the language.
One benefit of poetry is "fun" and "surprise" effect, which (together with rhyme) makes remembering whole sentences easier. Your brain is looking for surprise, or even better something sex-related, and filters out anything boring. The more vivid image you can conjure, the easier is to remember.
From personal experience, I used limericks, repeating them many dozen times, as pronunciation exercises, when learning English. Or lyrics of favorite songs.