Condition or emotion that is subject to change - Estoy enfermo. If it would be unclear what the subject of the sentence is, then the pronoun has to be included. The above lists of when to use ser and estar have to be memorized - using them incorrectly means you will be less likely to be understood, and people will definitely know you are not a native speaker. The same goes for the conjugations of ser and estar . Every Spanish verb has a conjugation, and memorizing them just goes along with learning the language.
La is the definite article that corresponds to feminine nouns - la casa, la mesa, la ventana .
Whether a noun is considered feminine or masculine is generally based on the last letter of the noun. The exceptions just have to be memorized as you come across them. When using nouns, you must make sure that you use the correct gender and number when using an identifier . The identifiers are el, la, los, las, un, una, unos, and unas . El and la are singular definite articles , which means you are talking about a specific thing.