The pomaceae-foliis palmatis subtus niveis of Quettah and Candahar are nothing but this poplar in its young state!
Subtus altare suggests a crypt, but there seems to have been no crypt under the choir.
On the pavement south of the altar is a piscina, which (if this be its original position) must have belonged to a chapel or chantry behind the high altar--possibly the chantry of the Holy Trinity subtus altare.
The above list will hopefully give you a few useful examples demonstrating the appropriate usage of "subtus" in a variety of sentences. We hope that you will now be able to make sentences using this word.