There are particular words that seem to show up in word problems for different operations that can tip you off to what might be the correct operation to apply.
Make sure your student reads the entire problem first.
It is very easy to start reading a word problem and think after the first sentence or two that 'I know what they're asking for...' and then have the problem take an entirely different turn.
Word problems are often a source of anxiety for students because we tend to introduce math operations in the abstract.
Students struggle to apply even elementary operations to word problems unless they have been taught consistently to think about math operations in their day to day routines.