I have come across this sentence
It will be hard to bear If I see a child who can't achieve his goal
.
I think It is always true that when you see a kid who can't achieve their goal due to lack of social support or their race you get sad and It is really hard to bear.
Can we use zero conditional in these situations?
The zero conditional is used for universal truths e.g.
'If you heat ice , it melts.'
It can also be used when the speaker believes that when the condition is fulfilled the result is always the same e.g.
'If I am even 5 minutes late for work, my boss shouts at me.'
It is important to understand that this 'universal' truth is entirely in the mind of the speaker and may or may not be true if, say, the boss was absent
The first conditional is used when we want to convey that, when the condition is fulfilled, the result, in the speaker's opinion, should be or is very likely to be true. E.g.
'If my boss finds me here at ELL, he'll fire me."
Coming to your example,
It will be hard to bear If I see a child who can't achieve his goal
If the speaker considers, "hard to bear" being universal truth when the condition "If I see a child who can't achieve his goal" is satisfied, you could arguably use the Zero Conditional.
However, if the reader of this sentence does not agree with you on the universal truth, it may cause confusion. So, First Conditional is the most appropriate in this situation.
