I am assuming you already know the basics such as word order, gender, number, etc. Note that Spanish word order is not as strict as that of English (e.g. sometimes the subject of a verb is at the end of a sentence).
- Make sure to know the personal "a" (i.e. Conocí a María = I met Mary).
- If you plan on spending a lot of time speaking with Argentinians, learn the "vos" conjugations.
- Learn the diminutive and augmentative suffixes (e.g. perro [dog] -> perrito [small dog]; bueno [good] -> buenísimo [very good]). At least in my experience, they are quite common, especially in informal speech.
It is important to be able to quickly conjugate verbs in any tense and mood. However, it is also important to be able to use the tenses and moods properly. Flashcards are good, but you need to make sure to speak, read, and write a lot. I am sure you didn't plan on only using cards, but too many people assume they will wait until they are fluent to do those things.