Learn German: Es hat geregnet, aber wir sind gewandert.

In German, the spoken past tense (Perfekt) usually uses a form of haben (to have). However, verbs that show a change of location or motion from one place to another, like wandern (to hike), use sein (to be) as their helper verb. That is why we say wir sind gewandert.

Why does the first part use hat instead of ist?

The verb regnen (to rain) does not describe a person or object moving from one place to another, nor does it describe a change of state (like waking up or growing). Because there is no travel from A to B involved, it takes the standard helper verb haben, making it Es hat geregnet.

Both geregnet and gewandert end in -t. Is this a pattern?

Yes, both regnen and wandern are regular verbs. To make their past participles, you take the ge- prefix, add the verb stem (regne- or wander-), and put a -t at the end. Many other German verbs follow this exact pattern.

Does the word aber change the word order in the second half of the sentence?