How to say "I like tennis" (etc) in Spanish
A sentence such as I like tennis, he likes cats etc looks simple enough on the surface. However, it turns out that to say the equivalent in Spanish requires a few bits of grammar that can be tricky for the beginner, particularly for those who haven't learnt any other language related to Spanish.
The first problem is that this is one of a number of expressions that Spanish tends to encode 'the other way round' to English. In English, the experiencer or person having the feeling of "liking" is the subject of the sentence1. In Spanish, the usual verb to express the notion of "liking" is gustar. However, this verb works more like English verbs please or enthuse: the experiencer becomes the object of the verb, and the agent (in other words, the thing-- in this case tennis-- causing the person to have the feeling of "liking") becomes the subject. So in Spanish, to say "I like tennis", we actually use a construction more similar to "tennis enthuses me" in English. Since the agent ("tennis") is now the subject, this is what the verb will agree with. So just as with English enthuse, if the agent is plural (cf tennis and badminton enthuse me), then the Spanish verb gustar will also need to have a plural form.
Another difficulty in the Spanish sentence is word order. In Spanish, a special type of word is used whenever the object of a verb is a pronoun2. This special type of word is technically called a clitic. A clitic is something similar to English -n't (as in don't, can't etc): it gets "hooked on to" the beginning or end of another word. In Spanish, clitics are placed before the verb in many cases, a bit like in the English phrase to go house-hunting. So to express the idea of "I like tennis", Spanish actually says something similar to "tennis me-enthuses".
With all this in mind, these are the basic steps to translating a sentence such as I like tennis.
Step 1: the verb gustar
The verb gustar, which as we mentioned is the usual verb to translate English like, is a regular Spanish verb. If you're not too familiar with how Spanish verbs work, you might want to go through this site's interactive introduction to Spanish verbs. For now, we'll just consider the singular "he/she/it" and plural "they" forms, which are:
So if the sentence says that we like a singular thing, then we'll need gusta. If the sentence says we like a plural thing like cats, red shoes, then we'll need gustan.
Step 2: add the clitic ("pronoun") corresponding to the person
Step 3: add the "thing being liked", remembering the article
Now we translate our noun phrase such as tennis, cats or whatever. If there isn't already one there, we usually need to add the definite article (el, la, los, las) because we're expressing a "generality". So for tennis, we'd say el tenis; for cats, we'd say los gatos. Although it's the subject of the sentence, it's actually common to put it after the verb in Spanish. So this gives us sentences such as the following:
If you put the subject before the verb (which is also possible in Spanish), it must go before the clitic. Recall that a clitic is a special word that gets "glued on" to the verb, so you can't put the subject between the clitic and the verb. So that gives los gatos me gustan etc as an alternative word order.
On the next page is a tool allowing you to see some examples with gustar interactively.
Other similar verbs
Some other common verbs have Spanish translations that use the "reverse" construction to English.
1. The subject of a sentence is the part of the sentence that the verb agrees with.
That is, the verb like agrees with I. Consider what happens if you
change the person to he: now the verb form must be likes to agree
with the subject.