Hi there, I know as an artist it could be a hassle deciding what supply could achieve a particular effect or work best in a particular situation. So this is going to be a guide series into art supplies, giving a brief description and stating the uses. Just to clarify this would be a summary giving just the key point which I've learned based on my experience using each of the tools stated.
In this part, I would briefly cover

• pencils
• pens

Pens

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Legends states they were made solely for the purpose of writing till.... Just Kidding! Normal writing pens are a great tool for us artist as they are hand held like the pencil. Though similar to the pencil they they don't erase and are ink based. Pen are of various types, I'll discuss based on the nature of the tip.

Ballpoint pens

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From the name, the tip of these pens are ball shaped(a lot of engineering going on in there). It is a pressure sensitive tool with the slightest of strokes shown on paper, thus more pressure gives a darker range of values when shading. For commissioned portraits where realism is key, ballpoint pens are very handy.
Uses: subtle shadings and gradation when needed

Fine tip pen

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Unlike ballpoint pens, they come with a fine tip, pointy in nature. The nature of this tip is solely for precision and thin nature of lines. They come in handy when doing quick sketching and rough sketching. Fine tips mean more details can be applied when needed.
Uses: outlining, detailing, sketching and writing

Pencils

Pencils based on type could be categorized into Graphite and Charcoal pencils. I wouldn't go into much details here because it's quite elaborate. I would do so in future posts.

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Pencil in general come in varying degree of hardness based on the constituent substance(charcoal on charcoal pencils and graphite in graphite pencils), thus the different grades H--HB---B(in decreasing order of hardness making B grade the softest). We artists make use of B grade pencils for shading, sketching and otherwise making the pencil a must have for every artist.
In conclusion, I strongly advise you interchange the pen and the pencil from time to time and see what works best for you. I love my pen more than my pencils based on experiences. So experiment experiment experiment.
Stay blessed and thanks for reading upvotes and comments are very much appreciated ;)