Continuous verb forms in Spanish
If you've learnt certain other languages such as French, you will probably be used to the idea that some languages don't always mark a difference between walks (a "simple" form) vs is walking (a "continuous" or "progressive" form). However, Spanish is actually more similar to English than, say, French, and often does make this difference. (For information on differences between how continuous and simple present tense forms are used in Spanish and English, see the later section on when to use continuous forms in Spanish.)
How to make continuous forms
Forming continuous verb forms in Spanish parallels English. We use one of the verbs to be plus the equivalent of an -ing form:
continuous form = form of estar + form ending in -ando/-iendo
For example, I am working would be estoy trabajando. The form ending in -ando (or -iendo for some verbs) is often called the gerund or present participle1. In this construction, it is equivalent to English -ing forms.
The gerund/present participle in Spanish
As noted, the gerund is the equivalent of -ing forms in English when used in the continuous construction. To form the gerund of most verbs in Spanish:
The 'same vowel change as in the preterite' generally means changing the e vowel to i for those verbs that have a vowel change (e.g. sentir > sintiendo). Many otherwise irregular verbs actually form their gerund regularly. For example, dar > (estoy) dando.
Here is the present continuous of the verb trabajar in full:
Next: practise the continuous forms
On the next page, you can practise the present continuous of -ar verbs.
1. Many authors prefer the term gerund, reserving participle for a form that has uses closer to an "ordinary" adjective. Because it appears to be the more common term, we'll stick mainly to the term gerund here. But arguably, the distinction is a little bit spurious. Generally speaking, including in both Spanish and English, participles function as both verbs and adjectives to some extent or other; really, we're just talking about "ends of a spectrum". The argument is that Spanish tends more towards the verbal use, and so "present participle" is not an appropriate term. But although less common than in English, examples do occur in Spanish such as productos conteniendo paracetamol, where the "gerund"/"present participe" heads a relative clause (so is to some extent adjectival).