How to use the program efficiently
I have been using the program and it is generally well, but I wonder if I am missing something. Is there a list of the words in Spanish and English and the grammar before we are tested on in each lesson, or do we learn merely by trial and error? It seems terribly inefficient, when a few lines such as "here is the vocabulary you will be studying and tested on" or "here are the verbs and the parts of speech you will be learning". Am I missing something?
That was the old Duolingo system, pre-crown era, before the "crown update" and rollout in February-April 2018.
Yes, exactly this.
But most people seem to only use the new "crown level" system (e.g pyramid or all skills@L5) and many users make the error to level their skills to the highest L5 level - as a form of a SR-replacement - too soon even when they have not completed their tree.
I sometimes stumble across users on Duome.eu who have learned maybe 15-20 or 30% of their total skills (course progress) and they already have ALL of them on the L5 crown level.
But they fail to push forward and all of the rest of their remaining skills are locked/grey.
Obviously they do not use the (global) PRACTICE button regularly, as they then would stay on the lower L1-L2 (L3) crown levels and use the other approach method.
We also need to be well aware, that:
1) we can only hit the hidden barbell "skill PRACTICE" button for L1-L4 skills here: http://www.duome.eu/USERNAME/progress
2) The "crown overlay UI web frontend" does only support the skill PRACTICE button for L5 crown skills on the www.duolingo.com web portal
3) that the global PRACTICE button requires many XPs per day/week as it focuses on a skill re-strengthen TOP to down approach; you simply can not reach your "recently learned" skills for 4h+12h or 1-6 days SR review intervals as you have to focus too much on the first half of the tree and as words decay each day, previously skills@100% strength are going to lose their strength over time and go back to 25-75%.
Do you know about the "Tips and Notes" behind the lightbulb icon on many lessons? That takes the guesswork out of the grammar.
For vocabulary, hover your mouse over new words to get a list of possible definitions. Over time you'll need to "peek" less often and the vocabulary will move from short term memory into long term memory. To do this efficiently, read the link on Spaced Repetition that J_J_max provided.
Consistency over time is the key to language learning. That, and using a variety of resources.