I can’t comment on Pali tenses, but can do so on the meditative aspects.
At the start, the meditator has to be just aware of whether the breath is an inhalation or an exhalation.
Then with the body and mind relaxing the breath lengthens (less hurried). With even more relaxation the breath is short, possibly due to needing less oxygen. These two steps happen just by being mindful of the breath, generally.
The short breath is now a suitable object for concentration, as it will allow for the focus of attention on a smaller field of awareness (the shortened ‘body’ of the breath). Not having to move the point of focus over a long breath makes for less disruption, and an opportunity to deepen samadhi even further.
For the entire body of the breath to be felt well on this short and therefore very transient object, an intensifying of the mindfulness is required, which I would now call concentration (in practical and English terms, not as a translation). This step therefore requires will and application of focus (sikkhati), that the previous two steps didn’t require. This concentration leads to a deeper samadhi.
This marks the start of what I would call ‘the Buddha’s mindfulness of breath’, which is an expert instruction on utilising the breath until release of the mind (first jhana), and avoids the meandering journey into samadhi. It is ideal, actually, for practitioners experienced in Mindfulness of breath, such as the gathering of Bhikkhus on the day it was taught as the ‘path’ into samadhi would have already been trodden well and this teaching offers a framework on which to navigate the mind into jhana as adroitly as possible. The teachings offers an glimpse into the the sheer expertise, sophistication and finesse of the Buddha’s skill in Mindfulness of breath.
The fourth step requires calming the body - this could be the physical body (more effective in my opinion) and/or the body of the breath. This added relaxation causes the cessation of the breath. A few seconds later it starts up again.
Jhana is reached at the 12th step (Vimocayaṃ cittaṃ- release of mind).
With metta
Mat