High Level Overview-
This is yet another great grammar topic that may not get the amount of responses it should get IMO for many reasons. You are assuming that most have done formal studies and many have not. Learning on the internet has somewhat replaced formal classroom studies for many which is unfortunate because classroom studies for languages with a good educator is a combination for success. If you find good sources on the web they can compliment what you already know. However, web sources are limited and tend not to be extensive from what I have seen. I am a strong believer in purchasing (or borrowing from the library) proper textbooks and formal studies.
In your post you stated the subjunctive tense however to be correct the subjunctive is a mood. There are three moods in Spanish, the indicative, subjunctive and imperative mood. All textbooks provide a definition of the three; therefore I will not extend here. However, part of the difficulty lies in the fact the subjunctive has virtually disappeared in English, and many people will say that in English it does not exist. Sure it does, most times people err or the verb form does not change so speakers who are forced to learn the grammar of another language tend to say the subjunctive does not exist in English.
Commonly today "I wish I was rich" has replaced the correct grammatical form "I wish I were rich". Since it is a contrary to fact statement "were" is the correct verb form in the subjunctive. If you say "I was rich", that clearly means at one point in time you were, it?s a statement of fact. However, to use "was" in an "if" statement is incorrect since you are not rich. There are many more examples of incorrect usage in English which definitely impacts the usage in another language where the subjunctive is vital.
Out of the languages derived from Latin, Spanish has the strongest usage of the subjunctive today followed closely by French. Surprisingly, some of the subjunctive forms in French have been lost but for the most part it?s just as alive and vital in this sister romance language. It?s so wonderful to be able to compare and in general there are similar patterns in Spanish and French but there are grammatical patterns in Spanish that require the subjunctive where French uses the indicative which is an example of the extensive usage in Spanish.
I think you should specify the areas of grammar that require the subjunctive that pose difficulty. It?s a broad topic. Certain verbs, adverbs of time referring to the future (cuando tengas tiempo, cuando vayas a la tienda etc.), prepositional phrases, most forms of the imperative mood require the subjunctive (no vayas, no digas, no dudes etc. ) and many other instances. Understanding why is the key to correct usage and practice. Basic knowledge is not enough because when you get into complex sentence patterns knowledge of the subjunctive is necessary. Espero que, quiero que, es necesario que followed by the subjunctive is basic grammar.
As I keep saying in this forum, to master Spanish and any language, other than essential good grammar knowledge reading is so important. If you read good newspapers, magazines and books combined with grammatical studies you will improve your understanding of the subjunctive and its importance from a grammatical perspective. Day to day interaction alone will not suffice to master the usage.
There many excellent resources on the market if you are willing to spend some money on a good textbook. I am not sure how much I will participate in this thread because I would like to see others participate so if you are looking for more advice and suggestions feel free to send me a private message.
-LDG.