1. Looks like gtk_text_iter_begins/ends_sentence work incorrectly with '!',
'?', '\n'

Test case:
#include <gtk/gtk.h>

void starts (const char *str, gint pos);
void ends (const char *str, gint pos);

int
main (int argc, char** argv)
{
    

    gtk_init (&argc, &argv);

    printf ("1\n");
    starts ("Sentence one. Sentence two", 14);
    ends ("Sentence one. Sentence two", 13);

    printf ("2\n");
    starts ("Sentence one! Sentence two", 14);
    ends ("Sentence one! Sentence two", 13);

    printf ("3\n");
    starts ("Sentence one? Sentence two", 14);
    ends ("Sentence one? Sentence two", 13);

    printf ("4\n");
    starts ("Sentence one?! Sentence two", 15);
    ends ("Sentence one?! Sentence two", 14);

    printf ("5\n");
    starts ("Sentence one?! Sentence two", 15);
    ends ("Sentence one?! Sentence two", 14);

    printf ("6\n");
    starts ("Sentence one.\n Sentence two", 15);
    ends ("Sentence one.\n Sentence two", 13);
    ends ("Sentence one.\n Sentence two", 14);

    printf ("7\n");
    starts ("Sentence one!\n Sentence two", 15);
    ends ("Sentence one!\n Sentence two", 13);
    ends ("Sentence one!\n Sentence two", 14);

    printf ("8\n");
    starts ("Sentence one?! Sentence two", 15);
    ends ("Sentence one?! Sentence two", 13);
    ends ("Sentence one?! Sentence two", 14);

    printf ("9\n");
    starts ("Sentence \none!", 10);
    ends ("Sentence \none!", 9);

}

void starts (const char *str, gint pos)
{
    GtkTextIter iter;
    GtkTextBuffer* buffer;

    buffer = gtk_text_buffer_new (NULL);
    gtk_text_buffer_set_text (buffer, str, strlen (str));
    gtk_text_buffer_get_iter_at_offset(buffer, &iter, pos);

    if (gtk_text_iter_starts_sentence (&iter))
    {
        printf ("STARTS\n");
    } else {
        printf ("DOESN'T  START\n");
    }
}

void ends (const char *str, gint pos)
{
    GtkTextIter iter;
    GtkTextBuffer* buffer;

    buffer = gtk_text_buffer_new (NULL);
    gtk_text_buffer_set_text (buffer, str, strlen (str));
    gtk_text_buffer_get_iter_at_offset(buffer, &iter, pos);

    if (gtk_text_iter_ends_sentence (&iter))
    {
        printf ("ENDS\n");
    } else {
        printf ("DOESN'T END\n");
    }
}

Output of the compiled code:
1
STARTS
ENDS
2
DOESN'T  START
ENDS
3
DOESN'T  START
ENDS
4
DOESN'T  START
ENDS
5
DOESN'T  START
ENDS
6
STARTS
ENDS
DOESN'T END
7
STARTS
ENDS
DOESN'T END
8
DOESN'T  START
DOESN'T END
ENDS
9
STARTS
ENDS


I assume that these parts of output show wrong behaviour of the functions
above:
2: DOESN'T START
3: DOESN'T START
4: DOESN'T START
5: DOESN'T START
8: DOESN'T START
9: BOTH


Probably, this is a Pango bug.