This paper attempts to deepen our understanding of teachers' selection and modification of mathematical tasks for their classroom instruction. In this paper, we focus on teachers' orientations and resources that subtly influence teachers' choices during their routine undertaking of task selection and modification. Interviews using stimulus material were used to induce teachers to expound their thought process as they deliberated the suitability of tasks and made choices related to task selection and modification. These thought processes are critical as it provides potential explanation for teachers' task inclinations. The findings of this study show that whilst "beliefs of the teaching and learning of mathematics" and "beliefs of students' capabilities and capacities" are preponderant as the teachers deliberate on which tasks they would use, the orientations within these beliefs differ amongst them. These orientations, productive or unproductive, influence their decisions during task selection and modification. Teacher resource specific to mathematical knowledge for teaching (MKT) is also critical in impacting teachers' choice of tasks. Teacher orientations and expertise impact the range and extent the teachers draw on their MKT, resulting in dissimilar inclinations towards tasks for mathematics instruction. In addition, resources such as preparation time, curriculum time and materials teachers have at hand influence their choices when selecting and modifying mathematical tasks for instruction.