The present tense generally expresses one of two things: 1) what is happening right now or 2) something that happens routinely or often. In English, and a few other European languages (e.g. Spanish), there are two different forms to make this distinction. For example:

I shower every morning.

I am showering right now.

This is known as the gerund versus the simple present. In Swedish, we don't make this distinction. We use the same form of the verb whether we're in the shower getting clean right now or we want to defend ourselves against an accusation of a poor hygiene routine.

Jag duschar varje morgon.

Jag duschar just nu.

In both cases, the form of the verb is duschar, which is derived from the infinitive att duscha.

If the letter a is the common thread of the infinitive, the letter r is the one for the present tense. All active verbs in the present (except, again, for the lonely little deponent verbs) end in r. The four alternative endings are

1. -er, which is the most common
2. -ar, which is slightly less common
3. Deletion of the final -a from the infinitive
4. -r added to monosyllabic verbs

Is there a rule for when to use what ending? Yes and no. Points 3 and 4 are more rule-driven than 1 and 2.

Point 3 is applied to verbs whose last syllable is -ra, e.g. att lära, att köra, and att göra. That's why, rather than adding an extra r at the end, we simply just take off the final a.

Point 4 is for short verbs with one syllable (i.e. "monosyllabic"), e.g. att gå, att bo, att ge, etc. Here, we just smack on a final -r and call it a day.

For points 1 and 2, it's essentially a guessing game, albeit a heavily weighted one. -er is much more common, which is why I advise my students to work through the following flow when deciding on an ending:

1. Do either point 3 or 4 apply?
2. If yes, then you know what to do.
3. If no, guess -er.

Over time, you will of course learn which endings are to be where by heart.