Sentence:  More importantly , several of the more advanced of the less developed countries have found through experience that they must plan their own complex investment programs for at least 5 years forward and tentatively for considerably more than that if they are to <WSD>be</WSD> sure that the various interdependent activities involved are all to take place in the proper sequence . 
Meaning of the word marked with <WSD> tags:  have the quality of being; (copula, used with an adjective or a predicate noun)