Seldom do they go out since the twins were born Почему здесь используется do they go , а не просто they go

Always, usually, often sometimes rarely and never употребляются 
- перед обычными глаголами
- НО после am is are и модальных глаголов (must can и т д)

1.You must ALWAYS brush your teeth twice a day. (always)
You must always brush your teeth twice a day.
2.Is he ALWAYS on time for work? (always)

3.Karen SOMETIMES has cereal for breakfast. (sometimes)

4.Paul doesn’t USUALLY  listen to music in the evening. (usually)

5.The children RARELY help with the housework. (rarely)

6.Must you ALWAYS  play your music so loud? (always)

7.Sheila can NEVER park her car properly. (never) заранее