I’m struggling with the change of tense in same sentence

Bonjour John !

This question is very interesting, and actually (as often) a bit more complex than it appears :) I'm going to give you a detailed explanation here, but know that the simple gist is that on Kwiziq, we'll make sure to use "be going to" when we expect Le Futur Proche, and "will do" when we expect Le Futur Simple, unless hinted otherwise :)

So in French, the nuance between Le Futur Proche (FP) and Le Futur Simple (FS) can be defined on 2 different levels:

- Temporal distance, i.e. how far away in the future is the action taking place. It applies to the 1st sentence you quoted:

Here both actions are yet to happen, but the action of eating will take place a certain time later than the action of giving the eggs, hence the FS "mangerai". The action of giving will take place much closer to the time we're speaking at (i.e. probably that same morning), which explains the FP "va m'offrir". So to sum up, both actions are in the future, but one closer to now + earlier than the next, hence the use of 2 different tenses to mark the difference.

- "Level of intentionality" - This is a more complex aspect of both tenses, and it goes as such: Le FP tends to express more personal input/intent into the future action's realisation, i.e. the speaker has a part into making it happen; whereas Le FS usually expresses an action that will happen, with or without personal intention. 

Be aware that these nuances are not systematically consciously expressed, but they can explain why French Natives would tend to use one tense rather than the other one, even if they're not themselves aware of why they're doing it!

Here I come to the 2nd sentence you quoted:

In this case, temporally-speaking, these 2 actions are in a more distant future, so you could expect to use Le FS for both verbs, which would also be absolutely correct here: 
"J'aurai du mal à me lever mardi, mais je serai bien reposée.". 

But the reason a lot of French Natives are perfectly comfortable using le FP here, "Je vais avoir du mal", is that the action of struggling can feel more personal, more directly dependant of the self, so here the "level of intentionality" aspect can override the "temporality" aspect, hence why the original sentence is also correct French.

I hope that explanation helped clarify the nuance for you, and as I said at the start, know that the Kwiziq team tries to make it straightforward as much as possible ;) 

Bonne journée ! 

Bonjour John,

Do you mean the change from the future tense ( mangerai, serai) and the near future ( vais avoir) ?