50% of Spanish vocabulary is way different from English. What is more important, usually the differences are found in very common words which makes the trouble even bigger. No one could possible guess that the word ‘casa’ means ‘house’ or ‘coche’ for ‘car’. Also it is very easy to encounter one of the many ‘false friends’, that is, words that look alike in two different languages but whose meaning is not exactly the same. Sometimes they mean totally different things like ‘carpeta’ (file folder) and ‘carpet’ (alfombra) and some others (by far the worst situation!) their meanings are alike but not absolutely the same.
Apart from that, one of the most challenging Spanish Magazine areas for English speakers is the gender of nouns. All nouns in Spanish are either masculine or feminine and depending on that corresponding articles and adjectives have to be chosen. An example: ‘el cielo’ (the sky, masculine) and ‘la tierra’ (the earth, feminine). Some can even have both roles and have to be differentiated through the use of the accompanying article or adjective. So, we can use ‘el estudiante’ to refer to a student boy or ‘la estudiante’ for a student girl. Generally speaking, masculine nouns end in ‘o’ and feminine nouns end in ‘a’. However, this is not always the case and some of them are not regular. Once more, the exceptions affect very usual words, making it hard to easily recognize them. Here are some very common ones: ‘la mano’ (hand, feminine), ‘la foto’ (photo, feminine), ‘el día’ (day, masculine), ‘el problema’ (problem, masculine).
To end up, there are many widely used variations and derived terms to express certain nuances. For example, words ending with ‘ito/ita’ will indicate something small or alternatively something for which we show affection. You can see this effect in ‘la casita’ (the little house) or in ‘mi casita’ (not my little house, but my beloved house). Words ending in ‘azo/aza’ will indicate something big like ‘la casaza’ (a very big house). Similarly, words ending in ‘ucho/ucha’ would denotate disdain like ‘la casucha’ (a hovel).
In Spanish there are two verbs for ‘to be’: ‘Ser’ and ‘Estar’. You can read the explanations in a grammar book and understand them very well but it will be a long time before you will stop making errors when you speak. The usual explanation is that ‘ser’ is used for permanent things, for features that define the particular object you are talking about and ‘estar’ is used for temporary states. Choosing one or another can really change the meaning of a sentence. To give you an example look at these two sentences in English. ‘I am boring’ and ‘I am bored’. The first means that you are a boring person and the second means that you are temporarily bored by the situation you are in. These two sentences translated to Spanish would be ‘Soy aburrido’ and ‘Estoy aburrido’ respectively. For possessions you use ‘ser’, eg. ‘es tuyo’ (it’s yours) and for position you use ‘estar’, eg. ‘¿Dónde está el coche?’ (Where is the car?).
The Spanish language has a very long history evolving from latin since the Middle Ages. That has made it collect many sayings stemming from the popular wisdom. Even we, natives, don’t know all the existing sayings, or are even able to interpret them correctly. However there are many that most of us know by heart and use in everyday life. ‘Poderoso caballero es Don Dinero’ or ‘Marzo ventoso y abril lluvioso hacen a mayo florido y hermoso’ are just two of those, the first one meaning: Mister Money is a powerful knight, and the second stating: if mars is windy and april rainy then may will be full of flowers and beautiful. Many times this type of rhymed sentences are the subject of conversation or are used to justify or argue a point. You can find lots of examples in the internet just typing ‘refrán’ at the search engine.
Same as in any other language, what people say in the streets is not the same as you can find in the newspapers. People tend to use more familiar constructs and words in day-to-day situations than what is considered as standard Spanish. The main difficulty with this is that this jargon evolves very quickly. Words are created and forgotten each and everyday. And even more, the evolution of slang is different not only in each Spanish speaking country but also on a region per region basis. A person speaking slang will be very likely not understood by other Spanish speaking persons not living in the same country.
In contrast with the easy and extremely regular verbal tenses of English, in Spanish we find a great variety of verbal forms. Not only do we use more verbal tenses with different endings for each pronoun, but also the chosen tense might as well be used to convey a different meaning or our opinion on the things we talk about. Also, the subjunctive is used a lot in Spanish but not so much in English. For example, one of the most common uses of the subjunctive in Spanish is with ‘want’. An easy structure for a foreigner learning English could be: ‘I want you to do me a favour’. In Spanish you would have to say (literally) I want that you do me a favour (‘Quiero que me hagas un favor’). ‘You do’ would normally be ‘haces’ but because you have to use the subjunctive you have to change it to ‘hagas’ which is the subjunctive form.