Tense:
Being a time agent isn't like being a Time Lord. Jack has had to learn the language in every time period he's been to, on every planet. He got used to changing languages without thinking. And now he's been speaking the same Old Earth English for over a century. Longer than he's ever used any language. Longer than he's ever used his own.
But some things never change. He understands it fully, he can speak it, he can even have long philosophical debates in Old Earth English - and after all, you never really know a language until you fail to understand philosophy in it... but when he dreams, he still dreams in Boe-Shane.
And now that they're both locked up here, in their cell, the alien who speaks millions of languages and himself, he speaks to the Doctor in his own tongue, the language that's been dead to him for years and years, a language fit for a time travelling society.
The main distinction in Boe-Shane is one of tense: what was, what is, what will be. But it is not the crude perception of pre-time travelling humans, those who blunder into their future, unknown. For the time traveller, the future, like the past, is a matter of records, of certainty, of knowledge. What was and what will be are one and the same, a distinction made for the comfort of semantics. Only the present is unknown.
The Doctor, old and frail, is leaning on his, drawing heat from his body in the cold cell, and talks. All the time he talks. In his great knowledge and great understanding, he reassures Jack. He talks of how Martha will defeat the Master, how the human race will prevail, indomitable, how they will all get out of here, alive and well, and none of this will have ever happened, and all the while he speaks with the certainty of the future.
And as he is lulled into sleep by the rhythm of two heartbeats, Jack isn't sure what's the bigger comfort - these facts of the future, or that when the Doctor talks about his relationship with the Master, he always uses the past tense.
Aspect:
She called him 'pompous' when he told her the name of his species. But it's true. Time Lords were arrogant bastards, and if he didn't remember it before, the Master made sure to remind him of that in the last months. But they were right - they were the Lords of Time. It's built into their physiology - seeing time, no, feeling it. What could be, what must not. The devils that can be fought, and the devils that must survive forever, untouched. It's inherent to their world, and therefore inherent to their language. Some points are fixed; others are in flux.
He's been talking only in Gallifreyan these days, ever since the Master captured them. He can't really try and cheer Jack up, tell him all those lies people tell when they know hope is slim - but Jack understands. This is the comfort in Jack - he's also travelled in time, he's older now than most humans will ever be, he understands. He never complains. He doesn't complain when the Doctor raises false hopes, for his own sake as well as Jack's. He doesn't complain when the Doctor talks of possible futures, futures in flux, futures in which Martha wins, the human race comes out triumphant, everything is undone. Even Jack's death, one day, is a possibility now, with the paradox machine twisting the universe, mocking everything that is right and good and should be. Even he is in flux.
Only when the Doctor speaks of his love for the Master does he speak of fixed points.
