Learn Italian: Bisogna che tu tagli il prato adesso. - It is necessary that you cut the lawn now.

The phrase bisogna che (it is necessary that) is an impersonal expression showing obligation or necessity. Impersonal expressions like this are classic triggers that require the verb in the following clause to be in the subjunctive mood.

Wait, isn't the normal present tense 'you cut' also tu tagli? Are the indicative and subjunctive forms the same here?

Yes, they are exactly the same! For regular -are verbs, the present subjunctive ending for all singular subjects (io, tu, lui/lei) is -i. Because the normal present tense (indicative) for tu also ends in -i, the two forms are identical. You have to rely on context, like the trigger bisogna che, to know it is the subjunctive.

Since tagliare ends in -iare, shouldn't adding the subjunctive -i ending make it taglii?

It would, but Italian drops the extra 'i' to avoid having two of them in a row. The stem of tagliare is tagli-. When you add the subjunctive -i ending, they merge, so it is simply written and pronounced as tagli. This rule applies to all -iare verbs, like mangiare (becoming mangi) and studiare (becoming studi).

Could I say to mean 'You need to cut the lawn'?