If the phrase is given as the sum of one and eight then expression is like that 1+8. It is known to be conversion of the word into algebraic expressions. Qualitative analysis is not measurable but if you convert it into the quantitative analysis or which is to be easily quantifiable then you are completed in writing an algebraic expressions. Numerical expression and algebraic equation is one and the same thing as which is quantify in terms of the numbers then it is known to be as algebraic expressions.
Sometimes it is much complex word problem is given and you have to write algebraic expressions then you have to go step by step procedure as first you have to analyze what the questions wants in the solution and what are the expressions that are easily frame into the quantitative form. Then you have to use your brain and your logical reasoning to frame the equations.
Let us take an example like there are 5 people and each one of them is using the pens in the class. In the sequence individually use the no of pens as 4, 5, 3, 1 and 2 respectively. So the formation of the word problem is very easy. You have to see the need of the problem. Writing algebraic expression is easy if you understand the problem well. Now select the first variable and assign the variable as x for first person and multiply it by the number given as no of pens as 4 as 4x.
After solving the equation initially now you have to evaluate algebraic expressions. Here the expression is very easy as something in the form of summation of 5 persons who are using pens in the class. It is then easily solved further if another question is there. You have to check the linking between the questions.
We have to frame and grab every single digit and consider it which is beneficial for our calculation. Sometimes the numerals in the question given are not relevant just to confuse so that you will consider it and loose the marks.
We should have a deep thinking during solving the problem. The good thing is that as much as you do practice over these kinds of problems you are more confident enough to solve similar problems as maximum times the problem pattern is the same but solution is different. It is very rare that complex equation will come in your exam.