Learn Spanish (Latin America): Ir a la iglesia es una costumbre muy antigua.

In English, we use the -ing form to make a verb the subject of a sentence, like 'Going to church.' In Spanish, you must use the infinitive form, so 'Going' becomes Ir. Using the Spanish gerund Yendo here would be grammatically incorrect.

Why is it a la iglesia and not just a iglesia like 'to church' in English?

Spanish requires definite articles (el, la, los, las) when talking about certain institutions or places in a general sense, such as la iglesia (church), la escuela (school), or el trabajo (work). English often drops the article in these everyday phrases.

Could I use vieja instead of antigua to say 'old custom'?

You would be understood, but antigua is a much better fit here. Antigua is used for things that are ancient, traditional, or have existed for a long time in history, like a custom. Vieja usually describes something that is physically aged or worn out.

Since this lesson teaches soler (to usually do something), how could we express this habit using that verb?