I have just (July 2, 2019) worked through level 1 of the first four skills of the Duolingo Arabic course: Alphabet 1, 2, 3, and 4. These four skills teach you eleven letters of the Arabic alphabet (about a third of them), a few diacritical marks, and about twelve words, most of them proper names (the goal being to read complete words in Arabic, not to start building up a basic vocabulary yet).

• Alphabet 1: The letters و waaw, ا ’alif, ي yaa’, د daal, ر raa’, and ز zaay. The diacritical mark َ   fatḥa (used to write the short vowel a). I now have a separate page on the Alphabet 1 skill, with explanations and examples.
• Alphabet 2: The letters ذ dhaal, ب baa’, ج jiim, and ك kaaf. I now have a separate page on the Alphabet 2 skill, with explanations and examples.
• Alphabet 3: No new letters or symbols. Some proper names, including names of some cities and countries spelled with the letters you have learned so far.
• Alphabet 4: The letter م miim and the diacritical marks ُ   ḍamma (used to write the short vowel u) and ِ   kasra (used to write the short vowel i).

This material is split up as follows:

If you are new to Duolingo, then it might be helpful to know that on the Web version (but not on the app), you can access notes that explain some of the new material. To find these notes, first click on a skill (such as Alphabet 1) and then click on the lightbulb icon.

Presumably, the remaining letters and diacritics will be introduced bit by bit in the lessons that follow. Some learners will enjoy the way the alphabet is taught in this course. However, there are certainly others who would prefer a more systematic approach, with sufficient explanation. For those learners, I would like to recommend my book Bite-Size Arabic, which you can purchase from Amazon.