Today we are going to compare two words:必需 and 必须. You can hear that these two words have the same pronunciation. However, they are different in their meaning and usage.
必需：Is a modal verb but it is usually used with this structure“所…的”
必须：Is usually used as an adverb to modify verbs and adjectives. It is commonly used to talk about abstract things like matters or affairs which must be done.
Sometimes, 必需 can be used with “品” to be a noun phrase, which is usually translated as “necessities”. But 必须 cannot be used this way. They are not interchangeable.