The Spanish conditional verbs unit could use improvement.
There is only one lesson in the conditional verbs unit for Spanish.
The conditional forms are not contrasted against other forms of the verb, so we don't learn to distinguish between conditional and imperative/subjuntive: one can simply assume that all verb forms are conditional to pass the unit.
Common verbs that are irregular in the conditional are not covered ( I think? ).
Multiple choice questions contrast irrelevant material. For example, this question http://i.imgur.com/69VxH1p.png asks you to translate "I would like to read more" from English, but the options only ask that you be able to distinguish the meaning of the infinitives "obtener" "partir" and "leer". These verbs aren't even the part of the sentence that is in the conditional form. A more effective contrast might be to choose between "gustaría", "gustará", and "gustara", which are all forms which look very similar and might be confused in practice.
A lot of the verb tenses are covered near the end of the Spanish tree, I think 7 of the last 9 skills, and some are not covered in depth, as you mention.
I'm retracing my tree right now, on the last four skills today, and I'm supplementing the Duo lessons with other websites. I did the conditional yesterday and these links are what I have in my notes as additional useful info, if this will help anybody:
http://www.studyspanish.com/lessons/conditional.htm (links to quiz at the left)
I definitely agree that your suggestions for testing knowledge of verb forms would be an improvement. Conditional tense is super easy to conjugate, with minimal irregulars (same ones as future), but when to use it can be much trickier for learners.
I assume they want to at least expose students to all the important verb moods and tenses, but they probably figure that if you move to Immersion or use Spanish outside the DL framework then you'll quickly be exposed to many variations 'on the fly' and get practice there.
I try to read a news article every day at http://www.bbc.co.uk/mundo/ and when I started I realized quickly that even the five simple tenses would not be enough to let me follow along.
Yes, contrast would be great. However, I believe that Duolingo might be taking this approach so that others learning Spanish won't be easily frustrated. Great view on the verb "gustar" Happy learning! :-)