* Would you please explain the grammatical differences between the sentences in each pair of sentences below? 1a. We were surprised about the variety of species in this area. (Correct) <-- "were surprised" = passive voice of the verb "to surprise someone" 1b. We were surprising about the variety of species in this area. (Incorrect) Sentence 1b should be either of these: - The variety of species in this area was surprising to us. <--- "was surprising to us" = adjective - The variety of species in this area surprised us. <-- "surprised us" = verb in active voice + direct object Sentence 1a and the correct version of sentence 1b mean the same thing. 2a. Investigation of this issue is so hard. (Correct) 2b. Investigating this issue is so hard. (Correct) Sentences 2a and 2b mean the same thing. investigation of this issue = NOUN + of + NOUN PHRASE investigating this issue = GERUND PHRASE = VERB GERUND + DIRECT OBJECT A gerund phrase acts like a noun.Notice how the following two sentences mean the same thing but use different grammatical elements. 3a. I like to play video games. = I like + VERB INFINITIVE & DIRECT OBJECT (to play video games) 3b. I like playing video games. = I like + GERUND PHRASE (playing video games)

We were surprised about the variety of species in this area. (Good) We were surprising about the variety of species in this area. (Grammatically incorrect) These two sentences are both correct, and mean the same thing: Investigation of this issue is so hard. Investigating this issue is so hard.

However, you could fix the second sentence to say: The variety of species in this area is surprising.

Thank you Peachey for your useful link.

Pedram, this should explain the difference (which is quite huge): http://www.ecenglish.com/learnenglish/ed-and-ing-adjectives-describing-feelings-and-things/